how seriously should I take a smartctrl warning about possible HD failure?
 

I installed a fresh copy of my distro, and took note of something I see now and then. At the end, a message said something to this effect: the hard drive has passed the smartctrl test, but is more likely than average to fail in the next year. I have a new laptop hard drive, but would rather not break it in unless I have to. Should I probably dump the current hard drive as soon as possible, rather than trying to string it along a while longer? I haven't seen any actual problems.

Were I in your shoes I would go forward with using the current hardware, but make sure that I have backups of my critical files and a list of the software I use. Keep an eye out for a deal on a replacement drive and plan to swap out the old one and reload sometime in the next year.

I might also invest some time in SMART testing the drive and verifying the results a bit. The WORST thing you can do is ignore the issue, but SECOND worse is panic and replace perfectly good components for no reason. Verify and prepare, and you will be able to adjust to the drive failing in a month or lasting three more years.

I'd get the OEM diag suite and double check.

I'd read the numbers smartctl provides and make a judgement from that, not some "consumer-friendly" (read: phrase it so that they buy a new one asap) short message.
http://dt.iki.fi/hard-drive-and-fs-health-checks

that is, after making sure everything is backed up.

The SMART status of your drive(s) is just an indicator of their likely health status. It's the numbers in the "RAW_VALUE" column that matter. That's going to be your best indication that something maybe wrong/failing.

Wikipedia has a table for the SMART "ATTRIBUTES", and what they mean.

I had a drive fail on me, but smartctl still reported that it had PASSED the SMART assessment anyway. The numbers in the "RAW_VALUE" column told a different story. You can see it here.

very seriously if it is using the SMART off hdd. I had smart tell me my hdd has a few days left but that too was a biogas message as it already crashed.

Sometimes smartctl isn't perfect or can give you a false sense of security

Usually failing drives are best with visual or audio warning signs, I found this link online and it may help a bit

https://www.lc-tech.com/pc/warning-signs/

Excerpts from link

If seeing the warning when installing a distro, I keep going, it's been months since I got that warning, & nothing untoward has happened.
If it is telling you every time you boot, I'd get ready to replace it.

Yeah, I heard several clicks just before my drive finally died on me. Then files started disappearing from it, and then the partition table on it collapsed altogether.

There are two types of parameters in the smart test, Pre-fail and Old_age. If any Pre-fail parameter falls below the threshold value, smart will fail the drive, otherwise the drive will pass. You can read the results of the smart test directly by running:
assuming you have only one drive and it's designated sda which seems likely since this is a laptop. Also, since the smart test was run during an installation, I assume only the short test was run. You can run the long version if you want by running:
This will usually take 1 to 2 hours to run, depending on the size of the drive. It will give you a time to completion when you run the above command. After the time has run, you have to rerun the smartctl -a command to get the results.

Given the smart message you got during installation, you know none of your Pre-fail parameters fell below the threshold value and the drive passed. I'm guessing your hard drive is just really old as evidenced by one or more of the Old_age parameters and that's why you got that message. I'd just keep an eye on it and watch for signs of hard drive problems like unusual noises, I/O errors, etc. I've had old drives that run forever without failing and I've had new ones die after two years; sometimes you just get lucky. Nevertheless, keep backups of your critical data just in case.