Flora, at the things I go to, the answer would usually be "straight away". There are often minor variations in tunes from player to player but things usually fit together all the same. If you did find your version does not fit in with the rest or that you have mistaken one tune for another you can always stop playing (and be wiser the next time the tune crops up). Sometimes you might only know one tune in a set and there's no reason why you can't just join in just for that one.