I remove my outer cage once the lowest limbs are roughly 5 feet off the ground. This leaves plenty of the tree beyond the reach of the deer to survive any browsing it may get. I leave the "cage" on the trunk to protect it from rubbing and chewing from rodents essentially forever. Not sure it's required, but as soon as I do - my luck is that a buck will come buy and rub the crap out of it and kill it.