I think it depends on how the sun is where you are.
I live in Arizona, and we get a blazing hot sun in the daytime. My beardie can go from cold/sleeping to wide awake and energized in just 20 minutes by laying on a basking rock in my backyard. During those 20 minutes, I watch his color drastically change - he darkens when he first gets exposed to the sun, I think the shock of the real light kind of freaks him out. Then after about 5 minutes of dealing with it, his color turns bright yellow and beautiful and he becomes very attentive and watches everything around him. After about 10 minutes of being bright and alert, he's ready to jump off the rock and sprint around in the shade.
So in my experience, I would say about 20-30 minutes if you have a bright, hot sun where you are. Maybe up to an hour if it's cool and overcast. Just BE SURE to keep an eye on him the entire time! My dragon has an uncanny ability to watch me and wait until I turn my head so he can dart off his rock and go hide somewhere. The backyard gives him a lot of courage