How much time for how much text?

It depends on the person speaking. Your best bet is a clock and reading the speech out loud.

There are lots of variables to that question. I hope you’re not just going to post slides of the text and then read it out loud to the audience while they read along. Different concepts may take longer to present as well. I think the best way to judge by far is to time yourself during the delivery. I think the very rough rule of thumb is 1 page per minute though isn’t it?

A good way to get a sense of this is find the transcript of a presentation or speech you like, and then watch (and time) the video. The transcript and video of Obama’s speech on race, for example, are both easily accessible.