No set time to switch a dog from rag to tug to sleeve. Everything is up to the dog. There are also 4 different type (level) of sleeves to advance to as well. The first one being puppy...the last one a trial sleeve. They are harder as they move up in level. Everytime you teach something new....like the bark and hold - usually you move down a sleeve - makes it that much more enjoyable for the dog.
I found out the hard way...when a handler or helper is in a hurry to move to sleeves...it can put you back months in training. A dog can't even trial for a SchH 1 until they are at least 18 months old......no sense in hurrying things along