Hi @Eric
I'm not sure if a reliable metric exists as a guideline to answer your question. Even if it does I'm not sure I'd take too much notice of it; there are potentially too many variables that are likely to impact it.
If it were me, I'd be inclined to put myself in my audience's shoes and consider things like, word count, message complexity/density/familiarity, audience education/literacy skill, concurrent on-screen audio (music, narration, volume), concurrent on-screen activity/animation whiz-bangery, likely physical environment distractions, overall presentation duration (attention may flag over time), is this an isolated presentation or is it in the middle of a longer session...
After objectively considering stuff like that and wearing your audience persona, I think a realistic number will present itself - should certainly get you close enough for the pilot testers to not be too grumpy!
PeteB