Savannah Super Cat
A minimum time for a food transition would be 5 days, and with a kitten I would double that. It also could be a lot tougher than you imagine to make a switch happen at all, and might take 4-6 weeks and 2-3 varieties of food before they start gravitating towards one. So most importantly plan on having some failures because someone else is going to have an opinion that's out of your control.
Until when are they considered "kittens"? She will be 5 months old when I pick her up from the breeder.