House rabbit society actually says 12 weeks for veggies
I was feeding unlimited pellets, but my vet recommended limiting them somewhat due to Gaz having weight issues (still more than their adult rations would be, just not unlimited). sherwood forest's baby feed looks really good (
https://www.naturalrabbitfood.com/rabbit-food-labels/ ) though I've never fed it myself as my bunns were right at the 12 week mark when I learned about sherwood forest.
definitely find out what hay the bunny is eating before you get it - I got my bunns and found out that alfalfa is better, so that's what I fed them and little Gazzles got mild diarrhea from the switch because she'd been eating timothy before - I started feeding both to make the transition more gradual and she got better very quickly. since I already had the timothy from having to make a gradual transition, I've offered both ever since (they do eat mostly alfalfa, though).
around 7 months, you can start cutting back on pellets and transitioning them purely to grass hays (alfalfa is a legume hay; timothy, oat, orchard grass, coastal, bermuda, wheat, etc. hays are all grass and are all perfectly acceptable hays to feed an adult rabbit.)
