No, you can choose how often you volunteer.  There are many roles (turn up and try) that simply require you to give a few hours to make a difference, other event roles for example, will involve you for a defined period of time – like a few days and then you are free again until the next event comes around.  You can choose shifts and what events you get involved in.  Other roles will ask you to give a regular commitment, but you can always talk to the organisation about the time you have.