I started with about 20 minutes. There is a nice loop trail behind my house that's about 45 minutes, but there's a way to cut through the middle to half the time, so we did that. I built it up fairly quickly as my boy was already pretty fit, but it will depend entirely on your dog. Now we do 45 mins to an hour every morning. I could add more weight or time if I wanted to without issue but this is fine for us cause we do other exercise in the afternoon.
How old is your dog? How much exercise does he currently get? I would start him out with dragging something light like an empty milk jug just to get used to the feel of the harness and the feel/sound of something "following" him, then you should be able to add weight pretty quickly depending on his eagerness. My boy is a go, go go kind of dog so he took right to it.