You will love this for an indirect answer: depends on the conditions. Is there a current or tide - then you need enough wind to counteract them. Is there a lot of power boat/jet ski traffic - then you would need enough not to be a bouncing bob (unless you like that sort of thing). Otherwise, as long as you have a paddle and don't care how fast you go, the windless sky and your paddling ability is the limit.
P.S. For real light wind tie cassette tape to your bridle and shrouds. It will give you wind direction even in the lightest breezes.
