How long should I wait to tell a girl I love her if I have these strong feelings for her
That is a pretty strong word for a girl, first make sure she feels the same way before you can say that.
Don't Wait, Time is the Killer.
Pretty strong feelings may not mean that you love her. Give it a minute and see where the relationship/friendship is going. You also want to look for signs that she likes you a lot before you put yourself out there.