When they get 6 inches long you start thinking how bad it would be for a bird or cat to break off something so delicate! If this is a new stock in the ground I insert a metal or bamboo slim object (usually I use stainless steel rods, 3/16") very close to the stock and using masking tape in reverse, bind the new sprout to the rod as it grows (masking tape sticky sometimes harms young bark if it is actually sticking to the bark, hence using tape in reverse). That way passing kitty cats also cannot break it off in one of their turf fights they sometimes have.
Hope that helps.
[quote="Joe in 06807":zy82ctzl]I was expecting an answer which was more focused on leader length. A cat-based reference is not something I'm going to consider ...[/quote:zy82ctzl]
OK. When your clumsy friend is admiring your grafts and accidently knocks 4 inches of new growth off your rootstock by merely being .... clumsy... then you know you should have put a slender stake in the ground and supported that 4 inches of new growth!
You can "not consider" most anything you want, but most advice here is given from the school of hard-knocks! 
PS: you have been given leader lengths.
Your original question was TIME.
[quote="Joe in 06807":zy82ctzl]How soon do you choose a dominant leader on an apple graft.[/quote:zy82ctzl]
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