How long can you have swelling and/or water on the knee after surgery?

Varies. Most surgery swelling resolves by 6 weeks. But if you have arthritis, you can swell off and on forever. Your knee is lubricating itself more from increased friction.
It varies. Swelling on the knee after surgery is common. The length of time that you will have the swelling varies from person to person. Also, the type of surgery will factor in to the time frame. The more invasive the surgery (and the more severe the injury), the longer the swelling is likely to persist. Ice, elevation, therapy, and anti-inflammatories can all help diminish the swelling.