I'm assuming you are talking about how long the program remains programmed into the PIC.
I've been using PIC controllers since about 1989, or so. Still have instruments in the field that are running fine using them, as well. A lot depends upon circumstances, though. Using them in a heated environment, say upwards of 150C or more, will likely cause their programs to degrade much faster than otherwise. I've used them at 180C, in fact. But I reprogrammed them every month or so, just to be safer. (They aren't spec'd to run at those temperatures.)
I dont' know why you'd care to make them last much longer than they already will, but to make the programs last longer use a professional programmer to download the code. They will download the code with the right voltages and timing and will test the program over the entire specified operating Vcc range, to be sure about it. Then just don't abuse them.
Now, if you are asking about how long YOUR programs will continue running without crashing... that's a different question altogether. ;)