I heard they like to see at least 400 hours but it also depends on the type of hours. For example 300 hours working with a few organizations (ronald mcdonald house, habitat for humanity, environmental causes) looks much better than 800 hours at 5ks or something like that. As long as you have quality hours with something that you can show you're passionate about you'll be fine. Make sure to write about it in the secondary. Quality over quantity!