It depends on the school you apply to; some want it within a certain time frame (SUNY Downstate in NYC wanted them within the past 5 years). I did a lot of volunteer work as an undergrad (CPR/first aid instructor) and would have over 100 hours logged, but that was so long ago that they're not relevant anymore and I wouldn't know who would even be able to sign off on the hours.
You'll also want to look into the type of volunteer work the schools want, because Downstate wanted something that was non-clinical and "met human needs."
Best of luck! I'm sure you can find something that you're interested in.