As others have said "til it fails". I lose confidence at the 2-3 year mark. I do have some 6&7 years old that still work, but I wouldn't trust them with valuable data. My main rig is 3 years old and working fine. (Samsung 120g sata) But I will soon replace it just for peace of mind. BTW all 8 of my rigs run 24/7/365 folding@home.

Hi,

The MTBF for most modern hard disk sub-systems was approximately 10,000 hrs. The current MTBF for newer systems can be greater than a million. The quality and type will dictate the time frame. The general consumer PC hardware from a lot of the vendors is greater than that. So pick a drive manufacture an Google the specific drive. You will find the information there.
The drive environment will decide how long the drive really lasts. If you have a poor rated power supply and cycle often then the system will degrade.

If the thermal control of the system as a whole is poor then that will be another fact for th HD subsystem.

Now these devices depend on a lot of different factors just like the hard disk subsystem. If you have a power supply that is not designed properly and poor ventilation then you will have problems at some period in time. I would suggest that if you are just fishing to see what type of equipment to acquire in order to build a decent system. You will need to look at the amount of $$ you are willing to spend. It would be wiser to acquire a system built by a reliable manufacture.

The activity and usage along with the above statements apply here.

Back up schemes! Good power support; UPS (uninterrupted Power Supply) that is matched to the load properly.

This will depend on your needs and the OS that you are using. I've got systems that are still operating but could not be OS upgraded to what is currently available. But the equipment functions as a tool that it was designed to perform as.

A lot of the question you presented could easily been answered by doing a LQ or Google search.