There is no need to regularly drain the water out of the waterbed. Special waterbed conditioner known as an algaecide needs to be added to the bladder every six months to prevent the development of bacteria or algae. You may have to top off the water inside the mattress once a year.
The only time you have to fully change out the water in the bladder is when you move. A full-sized bladder takes anywhere from 1 to 6 hours to drain. If your waterbed develops a leak, you will have to patch it with a waterbed repair kit, which is a fairly simple process and can take around 30 minutes.