Emerald is right on, it totally depends on the environment. A properly maintained motherboard should last you quite a while. My folks have a rig that I build for them (and maintain for them) that has been running for about 8 years. I blow it out occasionally. It pretty much runs all the time.
One thing you don't really want is a case that is excessively hot. If you cram a ton of stuff in a case without the proper air flow, things will get hotter than they are supposed to, causing premature failure.