It depends on how often your basement floods. If it only floods during really bad storms, then maybe every 10 years or so. If it's flooding more regularly than that, then you should probably do it more often.

It depends on how often it rains in your area. If it rains a lot, then you should waterproof your basement every few years. If it doesn't rain very much, then you can probably wait 10 or more years before waterproofing it again.