In our home office we have 2 Macs, 3 Windows PCs and a FreeNAS system.
I don't do a lot of installing and removing of software on any of these systems, which is one of the biggest causes of OS based slowdowns, so for us, the main issue is the slowdown introduced by general clutter, updates and fragmentation.
Two of the Windows PCs are running Windows 7 one is running XP. The XP machine, with its 4 year old OS install is creaking now and in desperate need of a rebuild but I can't be bothered as the PC is a 9 year old P4 and used only very occasionally. The 2 Windows 7 systems, both around 18 months old and with the same build since new are fine. There has been a slowdown since new, but not significant and only really noticeable at boot time.
The two Macs are both running OS X Leopard (no real reason to upgrade at present and one is a G5 anyway). One is a 3 year old Intel based laptop that only gets very light use, it has slowed down since new, but again not significantly. The other is a 6 year old very heavily used PowerMac G5, which is used by my partner all day every day for her work as a designer. I rebuilt that for her about 2 years ago and it is now in desperate need of another rebuild as it has got painfully slow and very unreliable. Again, I've not bothered because she doesn't want the hassle and we know we're going to be upgrading it to a new one sometime in the fairly near future (if only Apple would make a model that suited her needs, we're waiting and hoping that the expected new MacPro may fit the bill). It seems to need a rebuild roughly every 2-3 years, simply because it slows down and becomes unreliable. As I say, the Intel one hasn't suffered this problem but then is not really used.
I fully expect the FreeNAS system to go on at the same level of performance until it breaks, but it's too early to judge yet.
In my work, I deal with estates of many thousands of Wintel machines (and occasionally far smaller numbers of Macs). I can say that as a general rule, in a well managed environment, somewhere around 2-4 years is typical before you start to see slowdowns on a Windows system, though W7 seems to be a little better than XP.
I'm sure either platform would go on indefinitely if you don't update and patch them, but that would be stupid.
Neither Windows nor Mac OS X seem to need defragmentation these days and if you have an SSD it makes very little difference as seek time doesn't really come into it. Just keep your disk less than about 70% full.
Rebuilding either will make them speed up, but adding all the patches and updates after the rebuild will slow them down again (though not by as much). In the Windows world you overcome this by slipstreaming the patches (updating the install media to include them during install rather than adding them after the event - handy in large envrironments but a pain for home use - it'd be much better if you could just download a new updated version of the install media). I don't know is whether you can slipstream OS X updates the same way you can Windows ones? Anyone know and care to comment?
So, in summary, any system that is used a lot and has had the same OS install for a few years will slow down, adding and removing software frequently is a major contributor to this and is best avoided, patches and updates are also a contributor. In my experience, Windows slows down a little more than Mac OS X does, however, Mac OS X seems to become more unstable.
The bottom line - if your system is slow or unstable and has had the same OS build for a couple of years, then it's worth trying a rebuild, whether it's Mac or Windows.
----------
Some interesting responses, I usually clean install after each year in college in order to get rid of all the garbage it accumulated during school use but seeing how good it is at staying clean, I don't think that would be necessary, especially since I now have an SSD and it would waste writes.
I'd agree it's probably a waste of time unless the system is noticeably slower or has become unstable, however, you don't need to worry about 'wasting writes'. Your SSD will very likely outlive the useful life of the rest of your computer, depending which SSD it is, it could theoretically outlive you.
