Hard one, I think it should be left until all the squad is finalised, and then it should not necessarily go to one of the longest standing players at the club, but the one with the best motivational skills.
This person can often be found, standing out on the training park and a coach worth his salt will be able to pick a couple out for the captaincy roles.
Some players do not need the extra workload of captaincy as it detracts from putting in 100% effort, but some just take it in their stride and do fine.