No, they aren't interchangeable - they're used to refer to different concepts. 'Une fois' is 'a time' - in the sense of an occurrence or an occasion. So 'la première fois' (watch your accents!) is 'the first time' and 'la dernière fois' is 'the last time'. And if you want to ask how many times someone has been somewhere or done something, you'd say 'Combien de fois..?' . 'Temps' refers to time in a more general sense, such as talking about how much time you spend on something or how long something lasts or takes. So if you want to know how long you'd need to do something, for example, you'd say 'Combien de temps..?' A rough guide would be this: If you'd say 'How many times..?' in English, this would be 'fois' in French. If you'd say 'How much time..?' in English, this would be 'temps' in French. Note that 'le temps' also means 'the weather' in French. I hope that helps!