You should leave a minimum of 72 hours between sends because it takes most people who will engage about that long to have meaningful results in teh data.
I would avoid resending on *just* opens, because opens depends on the lead's email viewer settings. Use does not click too.
Most times I've been asked to do this we leave about 1-2 weeks between sends.
I am also NOT a big fan of doing this because some people do see the email and get annoyed that you keep sending it. So try to mix it up in a nurturing program where you don't keep beating people with the same email each week.