We start checking ours when the temps hit 90*, or if it's super humid. We brought them in the basement when it was over 90* for a week straight. Otherwise, if you are home all day, you can dunk them in a bucket of cool water every couple hours.
My family raises Silver Fox Rabbits. They are very good meat rabbits because they are rather large, and have very nice pelts. We have outdoor hutches and just put plastic around the sides during winter, it's more summer you have to worry about. If it gets to hot you will have to dunk them in...