How long do I have to wait after I change the gaskets?

If you only changed a rubber gasket, then you don't need to wait at all. Just don't fly off. If you're using liquid gaskets alone or with rubber gaskets, it should say on the tube. Usually it can takes up to 15 hours for a liquid gasket to turn into rubber (so you can try running the engine after this time), but it may take up to 3 or 4 days to achieve a full seal. It also depends on the ambient temperature and how thick you make the paste (it'll take less time when it's hot outside, and thicker paste will take longer to fully cure).