I help out at dss' school one morning a week. I currently work 4 days a week, so it is quite a commitment/sacrifice for me too, but it is worth it. I love having an 'inside view' of school, and seeing what actually goes on, and getting to know many of the staff and children. We moved house/County 18 months ago, so I wanted to get involved, get to know people and children, and find out what was happening. I mainly do one-to-one reading. My motives are partly altruistic, but there's loads of personal benefits too ... for example I know what the 'average' reading ability is, and thus know how my children compare with their peers.
Our school have a policy, of which I approve, that you don't help out in your child's class. I would not want to do this, particularly with ds1. Last year I helped in the parallel Year 1 class, now I help in the year above.
You should be given some general guidance (safety, child protection, confidentially, etc), but if there's anything you're not sure about, just ask. Or if you're not sure what you can do to be helpful, just ask - the teacher will certainly point you in the right direction.
Schools understand that commitments change, and you don't need to sign up for life. If you don't like it, pull out. I hope you do go for it though, and enjoy it as much as I do. As I stated above I finally went back to work in November, but still wanted to continue helping in school as it is very important to me for all sorts of reasons.