Two numbers from one passage: how long it takes to read silently, and how long it takes to say out loud at a chosen pace.
To read aloud at the selected speaking pace: 0 min
Word count divided by the pace you selected. That is the whole calculation. Choosing 200 words per minute and pasting 900 words gives you about four and a half minutes.
The two paces are separate on purpose. Reading silently and speaking aloud are different activities, and a script timed at reading speed will usually run long in the room.
Technical material, unfamiliar names, tables, figures and quoted passages all slow a reader down. So does the reader's reason for reading: a person checking one fact does not read the page the way a person studying it does.
A spoken estimate ignores pauses, questions from the room and the space you leave for emphasis. Rehearse a presentation with a timer if the length actually matters — the rehearsal is more reliable than any formula.