It depends. The current Mac Pros tend to last a while, but consumers who bought the Power Mac G5 or the 2006-2007 Mac Pros got about 2-4 years before they were outdated, which is pretty average.
The pro Macs have stabilized, no huge architecture changes coming so far, and 64 bit EFI. But the Mac Pro can go out of date pretty fast sometimes. They're powerful, but if the underlying technology moves ahead (like when the PowerPC to Intel change happened), you've still got a problem. After the Intel switch, the Power Mac G5 only got one more OS update and then they were done.