Most often you also need to use your free spins within a certain period of time. Otherwise, they expire. Again, the exact validity of the free spins is depending on the online casino you choose to use. It is usually between one to four weeks. Then your free spins are considered to be exhausted.
One tip is, therefore, to only choose bonuses when you have time to log in to the online casino. Doing that you will not miss out on any good value bonuses!