While volunteers do not get paid and are providing a charitable service, it is no less a real commitment and obligation than a job. One of the reasons volunteering is such a huge help to nonprofit organizations is because volunteers meet real needs. Therefore, it is important that you show up when you are expected, that you do what the charity asks you to do, and that you work with the organization to create a schedule and expectations that are reasonable.
If you cannot undertake the kind of commitment that an organization needs, it is probably better for both of you to think about supporting the organization in a different way. If you know you want to volunteer, but do not have a schedule that allows for a regular commitment, there are several organizations - many based locally - that will match volunteers up with one-time opportunities. Try starting there, and develop your voluntarism as you can.