Well, I let mine be 8 years and 250 hours ago. I just heard a funky clunk in the rear end a few weeks ago, and right now it's tore down for a rear end overhaul. Differential bearings, bull pinion shaft bearings, and axle bearings are the minimum I'm in for. All the shafts and gears look good so far. Thankfully nothing is broken.
I'd make that choice based on a few factors: Do you pull it hard? Do you depend on the tractor for farming? How long do you want to keep it?