Could be a few weeks or several months. Really depends on the individual. There's no way to tell unless you get a Semen Analysis where they can check for volume and motility.
Deca seems to be especially hard on sperm production since it's a 19-nor, so if you're that concerned you might consider running the Clomid longer than 3 weeks. Also, Human Chorionic Gonadotropin (HCG) is helpful.
Are you wanting to make a baby, or just concerned about your sperm in general?