Stating a number of hours isn't possible. You have to use your people skills to kind of feel it out. I would say if you have shadowed them multiple times & they have a decent idea of who you are, it's safe to ask.
Just make sure you seem engaged/interested while you are spending time with them though so they don't feel like you are just doing it for the hours or a letter. They'll be much more receptive to going to bat for you, & likely write a stronger letter, if it seems like you are legitimately interested.