I recently returned from a prairie dog shoot. I shot 500 rounds of .222 Rem. Mag. over 3 days without a cleaning. Now my 50 grain V-Max seats .036" deeper into the lands than it did 6 months ago. When is it time to rebarrel?

When your accuracy goes south. As you've done, seat the bullets out a bit and add a pinch of powder. I wouldn't worry about it too much and I wouldn't be at it constantly. Assuming you weren't abusive in letting the barrel get too hot, you've got a long way to go before ree-barrel time.
I find that usually the groups get looser with an occassional flyer. You won't suddenly start spraying bullets all over the place. How loose you'll accept the groups is up to you. If the purpose of the rifle is to shoot P dogs and paper and it can hold under an inch with a flyer now and then pushing a bit out of an inch, you can kill a lot of sage rats.

Sometimes it's the luck of the draw. I have a 20+ year old Savage in 243 that I just changed to a 22.250 barrel last month. The throat on the 243 was eroded so much that the 58 grain V-max bullet I was using was jumping almost 0.5" before they touched the rifling and the barrel would still hold .75" moa but velocity had dropped from 3860 to 3720 fps.

Only 0.036" deeper ,no worries. Maybe top notch benchresters might change there barrels or rechamber then ,but I dont think that 0.036" will make much difference. Just add a touch more powder.(maybe 1/4-1/2 grain) and seat the bullet out to nearly touch the lands.