[Yosho]

If you do your regular maintance and replace parts when they wear out... a very long time.  Usually on Nissan motors, gaskets will fail before engine internals do as long as you've performed all the required service.  The problem is most people don't have the services done on time (or at all).  At a certain point it becomes cost prohibitive to pay someone to replace worn parts like a head gasket.  Now, if you can do the work yourself... that's another story.