some of it depends on where they normally live.
if they are typically indoor bunnies accustomed to climate control who just get trips outside to frolic in the fresh air, I wouldn't bring them outside when it's more than 80-85F TOPS. with my two, it didn't take more than half an hour in low 80s temps for them to start looking hot to the point that I would bring them back inside. dunno what your summers are like, but in texas it would be low 80sF in the very early morning and 100-110F by mid-afternoon, so we'd go outside somewhere between 6-8 am if we went out.
if they're currently outdoor bunns who actually have a chance to acclimate to the temps as things slowly warm up in the spring/early summer then I'd say 90-95F with precautions (shade, a fan across part of their area, misting their ears with cool water, giving them frozen water bottles to snuggle with, chilled tiles to lie on, putting ice in their drinking water, etc.)