If you're cooking chicken breast or thighs, 15 - 20 minutes. A whole chicken in a covered BBQ takes an hour, depends how big it is though - use a meat thermometer if you're not sure.
That is always a trick and here are a few tips to help with it. One is to figure out how well you wish to cook it. I would recommend that you use a good meat thermometer as you cook it and check it...