If you've ever volunteered your time, you are already aware that a charity will naturally want as much time and support as you are willing to give it, sometimes even more.
You can't blame them, but it's best to communicate clearly to set limits at the beginning in order to avoid possible disappointment later.
Be clear. Decide in advance how much time and for how long you want to commit to this fundraising activity, in order to avoid misunderstandings later on.