As heavy as you'd like
I've made a couple of 70 and 80# ash boardbows, and theres no problem with them. All you need to do is make sure there is enough wood for the job. A 68" long and 2" wide pyramid flatbow would easily draw 60# at 26". Flat belly, follow the grain, keep it dry (8% or below is a good idea with ash), and make sure to distribute the stress well along the whole limbs, and you should be safe.
