I have been a volunteer firefighter and Lieutenant combined for over 8 years with a little military service thrown in the middle. My dad has been one since before I was born and worked at the department that I volunteer at being as that it is a combination paid/volunteer dept. I also worked as a paid firefighter for a little while before some other opportunities came up. The best thing to do is like the other folks have posted. Go down there and start asking questions, most guys will be more than happy to fill you in and with the decreasing numbers of volunteers in this country over the past decade or so, they definitely want your help. They can let you know whatever the state, county/local govt., township/city, and department require of you as far as education, testing, physical constraints, time, etc. As far as time goes, it all depends on what training, fund raising, duty shift times, etc. that they have there. Different departments do things different ways. I know of some that require the fire fighters to respond to the station to get on the trucks and do not allow personal vehicles at a scene, and there are some like mine that the volunteers drive straight to the scene. So, long story short (too late for that), is to do like everyone else has said and go down there and ask as many questions as you can. It's much easier on them and you to find out if you truly have the time and desire before hand, than to get 6 months into it and decide you don't have the time or desire, then you have wasted both theirs and your time and money.
Good luck to you and hopefully you'll find your way to joining the brotherhood (sisterhood too for those that make me be p.c.).