Every month and every time after off-roading, dry or wet.
I had one go on the M62 last year .. it won't happen again. My fault, I knew it was bust, couldn't find a spare so took the chance to drive home, towing the caravan, wife and kids, from Anglesey ..
So did I, thats when my disco went in for service once a year but now I do minor servicing myself I also grease the props once a year, but then no off roading.
Like the rest of the guys,once a month if off roading,[keeps the U's well lubed and keeps most of the muck out].Otherwise once or twice a year or before a long journey..