There is no minimum life expectancy. I think the single most important thing you can do is to keep the computer running as cool as possible. Too much heat, long term, kills hardware. That said, the hard drive will die. Not a question of if, but when. So, make sure you have a backup. My preference is for a bootable clone on an external drive, from which if the internal drive dies, you can go right on working. There is also Time Machine. Some have both. Bear in mind also, that the PRAM (clock) battery -- usually a common CR2032 -- has around a five year life expectancy. It is deeply buried in the computer. After about five years Apple considers any Mac obsolete and won't work on it. You will then have to go to an Apple Authorized repair.
GetiStat Pro or Temperature Monitor. They are both free.
You will find a difference of opinion on this, but I use smcFan Control to manually ramp up the fan speeds. Many find that the system does not react to heat quickly enough. (Disregard anyone who tells you that your Mac will shut down by itself if it gets dangerously hot. This is true, but you want to avoid the kind of overheating that won't trigger a thermal shutdown.)
The way I use smcFan is to create four or five settings, not including the defaults, in increments of about 15% up to around 2K. To avoid burning up the fan motors, I use the minimum setting possible. Here is a thread with some temps, to get a very rough idea of the temps you may be looking for. In the winter, I run it at the defaults or just a few hundred rpm above. In summer, I go a bit higher.
https://discussions.apple.com/thread/2760644?answerId=13119267022#13119267022&messageID=13119267#13119267
I may also run a small fan aimed at the upper left of the back as you face the screen, the hottest area of the case. The aluminum back acts as a heatsink and keeping it cool will help to dissipate the internal heat.
With the computer shut down, periodically vacuum out the intake grilles at the bottom and behind the stand. This is where dust accumulates and restricts the air flow.
EDIT: Repairing Permissions is generally given as a step to resolve almost any problem. There are specific times to do that, but as general advice, that recommendation is pure voodoo, and a waste of time.
EDIT: Keep around 15-20% of the available drive space free. That said, I don't think this is linear. e.g. if a drive is 2TB, I don't think you'd need to keep anywhere near that percentage free.
Message was edited by: WZZZ