Years. Breaking it down to hours sounds like your a nitpicky person that lists every nice thing you do for someone, then hold it over their head later. ( "But I spent 4.7521 hours helping you with thaaaat." )
Three years is a nice simple number, shows long time commitment, and if they want more info at the interview, you can tell them about how much time per week in hours you spent volunteering. ("Im usually at the food bank for about 5 hours a week, sorting and helping with paperwork.")
Personally I would put 3 years because it implys that you've spent a long time and a lot of effort into that place. 3 years can = much >200hours. Its a subtle misrepresentation that works in your favour.