How long can a bunny stay out on a hot day? (read for details)?

Its summertime and this is probably the best opportunity to take my rabbit outside in the fenced in yard. Its been on average 70-80� lately and I wanted to know if he can stay out a while. Yesterday he was out for fifteen mins before his nose twitched a little faster. I thought it might just mean he was hyper but I didnt wanna take a chance; I read somewhere it meant heat was bothering him. Theres always one shadey spot he loves in the yard and random shade spots on the.grass depending where the sun is.