The museum where I work is a state agency, so we follow the state's retention schedule regarding records of former employees and volunteers. We dispose of volunteer records three years after the volunteer separates from service at the museum. One reason I follow this schedule is that in some cases the volunteer is deceased and I don't think it makes sense to keep the records beyond three years.
Another reason I get rid of these records after three years is because there might be some volunteers who separated from the museum on bad terms or perhaps might be a volunteer I really don't want back. So, if for some reason they decide they want to come back after three years I would make them fill out the application again. Some might get turned off by that. Plus, if a volunteer left on bad terms and perhaps years down the road they decide to file a lawsuit for whatever reason, if I have kept those records beyond the legally required three years, I believe they could be admitted as evidence. Admittedly, these are EXTREME conditions that I have (fortunately) no personal experience dealing with in the 10 years I have worked at this museum, but I do take this into consideration.
If you do have a retention schedule I was consider using that as a guide for how long to keep your volunteer records. I hope this helps.