Marketo only provides this detail in Revenue Cycle Explorer. If you have this, go in there and find the Standard Reports > Email Time Heatmap or something.
But keep in mind that your previous data, if not tested, is biased...it will show you when people have opened it, based on when you've sent it. So naturally people will open emails at 9am if you've been sending at 8;30. That doesn't tell you if that's the best time for that audience.