1. What is a hobby?
2. What is your hobby?
3. Why do you you do that?
4. How many stamps are there in your collection? / How many items are there in your collection?
5. How long have you collecting stamps? / How long have you been having your hobby?
Через косую черту написаны вопросы, (1) если мы спрашиваем о конкретном хобби (коллекционирование марок) или (2)любом хобби, о хобби в целом.