If your PSU came with your case, consider replacing it if it reaches the 3-year mark. For separately-purchased (retail) PSUs, even if they only have a 2-3 year warranty (I'm looking at you, Antec), then they will most likely last 5 years, if they are of at least moderate quality. (Something better than Deer/PowMax/Chinese OEM non-UL-listed crap.)
If it has Japanese or solid caps in it, and it has at least a 5-year mfg warranty (most have 7-10), then I would probably attempt to run it till 10 years.
But, to avoid downtime, have a spare PSU on-site.