I have an Acer Aspire 7715z laptop bought in 2010. It is four years old. The specifications are:
As it is rather difficult to purchase Linux compatible hardware I am asking here to avoid having my laptop failing without having a replacement ready.

Is it time to purchase another one considering both the hardware lifetime and the hardware?

it depends on you: what kind of apps are you using, is there any problem with it, do you miss something important (for yourself)?
Otherwise you need not change it at all. There is no "linux compatible hardware", almost everything can be used (of course, there are exceptions).

You may try to add more RAM, use SSD instead of HD, so there are possibilities to speed it up....

Hardware compatibility is an issue where Linux is involved, as there is a subset of manufacturers who don't disclose all necessary data for their devices making it very difficult to write a driver for such devices.

To name one:
At present I have this device, although it works, it is closed, and apparently, updates are not that frequent. It would have been far better, had I used a different device from a manufacturer who was willing to provide all the necessary data for its devices. Security vulnerabilities are a problem with such a situation, as effectively, one would be at the mercy of the manufacturer. This happens with Microsoft: if it doesn't acknowlegde there is a problem, the problem will remain unpatched for who knows for how long.

Besides that device, I have the synaptics touchpad. I would like to avoid getting a laptop with the same kind of touchpad. There is also, the integrated GPU. The latter used to fail to properly initialize when mode setting was included in the Linux kernel.

Clearly, as someone who has been bitten several times by Linux incompatible hardware, it should be understandable that I start thinking about purchasing a replacement well before the current laptop fails. I am opting to buy a Linux-certified laptop with Linux preinstalled. The reason is not because I cannot grab a Linux Live CD, boot it, and use debootstrap together with chroot to install a system the way I want it.

Like anything, hardware has a mean lifetime. Beyond that mean, statistically speaking, failure becomes more probable.

ADDED LATER:
Using an SSD and adding more RAM do not increase the CPU's processing power. Besides that, the cost of a decent SSD makes purchasing one not an economically viable option. Regarding adding more RAM, I don't think it makes sense to spend good money on a obsolete RAM technology: remember, here we are talking about DDR2.

Another reason is eye-fatigue. My laptop has glossy display that increases eye-fatigue.

I was suggested to look for a laptop at system76 as they sell Linux certified laptops.

Depending on if the laptop was sold as a consumer or business laptop 4 years isn't that old. I always buy business ones since the components typically have a higher MTBF and business laptops tend to have a removable wifi so you aren't stuck with broadcom you can swap in your preference.
Synaptics and Alps are the two major makers of touchpads, I used to have a preference for Synaptics but Alps caught up and they seem just as good now to me.
My laptops that I use every day (one as a laptop and the other as a server) are from 2008, my kids use laptops from 2006 and my other is from 2002. In fact I just bought the Dell Latitude D430 that I use day to day because I couldn't find any new ones I liked.
What are you doing that you need more CPU, maybe a laptop isn't the best choice? I use my laptop for normal things such as e-mail/web/Libre/OpenOffice so my 1.6GHz CPU is adequate.
My laptop had a 1.8" drive which were more $ than a normal 2.5" laptop drive so it wasn't that big of a jump to an SSD. They don't give the super speed increase poeple say they do, I can tell that when I install a new package its about twice as fast but day to day use it didn't make that much of a speed difference as Linux is pretty efficient as managing ram and caching.

*edit- and yes, its nice to have a replacement 'ready to go', but really, its impossible to know when something will die. You're latpop could die the day after you get it, or it could last for 10 years....its impossible to know.

I'd be more likely to get some cheap 'netbook' to have as a backup then buy a better laptop when it does die over getting something just due to the age of your current laptop and the guess that its more likely to die after time period XXXX....

System 76 at al. are fine if you dont mind paying a premium and getting less (apart from a preinstalled linux) for pretty much exactly the same hardware as sold by older bigger vendors.

The 'linux preinstalled' vendors dopnt make laptops, they just sell other peoples laptops which they rebrand.

As long as you dont need or want a standalone video card (which I'd gues you dont or else the intel video you currently haev would be seriously underpowered) and are careful to avoid some of the less linux friendly wireless chips, you should be fine with an intel CPU/intel video chip laptop. AMD CPU/GPU laptops should be OK as well, but they arent as power friendly so you will get less runtimes on batteries (and if that doesnt matter, why have a laptop at all?) and are more likely to have issues with video support with linux.

SSDs are cheap now, and in most cases will make a noticable difference to system performance.

No, they wont add to the CPU processing power, but they will decease boot time, write time, read time and heat output, while increasing runtmes on battery.

RAM can help as well, the more that can be cached the less hits on the HDD which can increase runtimes (only by a tiny amount) and speeding up response time to resume programs.

Clearly it's depend for what you use it.
I'm using a laptop 10 and more years old. 256mb of ram, 1.3GHz of Cpu and 20Gb of HD

Thanks for all your replies.

So, opting to buy a Linux certified laptop is not an option as that would mean getting less for paying more. Upgrading the HD to an SSD would only be a cosmetic improvement, while upgrading the RAM would cost more for an obsolete RAM technology.

Conclusion: buy from manufacturers who make durable and reliable hardware while not sacrificing performance.


Thanks.

SSD is some improvement, I can see a speed difference during some things and as someone else mentioned reboot is faster. But I rarely reboot and don't install new packages all that often so I don' get a lot of benefit from it.
Basically I'm just following the trend, all the new smaller laptops are going SSD and since they are not yet big enough to hold all my data I'm using my current ssd based laptop as a way to transition to the new norm. I moved all my data to my server and setup a sync system and as I find things that are used/important those go into the sync structure and come down to the laptop so the rarely used, keep it for reference are left on the server. The laptop I want isn't made so I've had to adjust myself a little bit.