From my own experience, I've not fed more than 25-30 pinkies before moving onto fuzzies. That's 15-20 single mouse feeds and then 5 or so double pinky feeds before moving up to fuzzies. They'll move onto fuzzies at 3-4 months or so on my feeding schedule but of course they're all individuals and grow at different rates so you have to judg it visually for your own snake...you can use my data as a rough guide though. Good luck,
Adèle