I would think if you can do a 2-week water fast, a 3-week urine fast will give no problems. It is best to expend as little energy as possible whilst on the fast, though. The body should get required nutrients from the urine, and from cells that the body will eat. The bad, or damaged cells get eaten first - which is why fasting is so effective.
Fasting cycles (2-4 days) can also be very effective - but maybe with candida it is best to go longer.