Honestly it shouldn't take long for him to adjust if he is already well mannered. Pretty much all you are going to do is get rid of his breeding ability and high testosterone. (testosterone levels can take 1wk to 1mth to drop from stud mode to gelding.) Most stallions at his age already are going to have a set personality / temperament though. So if he's a good stallion he will be a great gelding. If he were a bad stud, chances are gelding him will take care of the testosterone, but you are still going to have a bad horse. (no manners)
As far as AI goes, you don't need a stallion, you can use him as a gelding and have the vet give him a dose of testosterone for the season. They make excellent teasers, but not as much attitude and no chance of accidental breeding. Once she shows heat you should have your vet come out and ultra sound her to see what her ovaries and cervix are doing. This is the only real way of knowing when the best time to send for semen is. She may be showing heat but that doesn't mean she's ready to breed. AI can be a very costly procedure, but It is well worth it for many people. If you spend more money getting this done then you will spend less in the long run. Then if you don't catch her at the right time and have to ship more semen.
Oh, and you could use your colt to tease her as long as he is starting to notice mares. So It really would depend on his maturity at the time. Other then that he's not going to care about who the mare is, she's in heat!!!
This is all coming from past knowledge, via my boyfriend.. He is a professional equine reproduction specialist. So If I am a tad off on my vocabulary that's why...lol