This depends very much on your treatment. It can be just one day for some dental treatments, 3-10 days for eye surgery or a 1-2 weeks for plastic surgery. However, some treatments need more check ups and longer rest. Pre-operative examination, surgery and final check up are usually done within one stay, so you only fly to Prague once. But once you are in Prague it would be a pity not enjoying this wonderfull city a bit.