There are two things to be aware of about beta readers:
(1) they read all of what you give them; and
(2) they read with the idea of giving you feedback.
Agents do neither.
I stop when I've decided the query or the pages don't work for me.
I don't read carefully, parsing out what works and what doesn't. I read your query and pages to see if I want to read more. I'm not skimming, but I'm not reading with the idea of giving feedback.
In other words, if you don't catch my attention I don't spend any time figuring out why, I just pass.
The point where people stop reading is entirely subjective. There's no way you can know where that point is for each and every agent.
Your take away from this: on the final pass, ask your beta readers where they'd stop reading if they were just reading for fun.
And if your beta readers love your work, but you're not getting requests for fulls, find new betas to read as though it was just for fun, and then see what they say.
Bottom line: find betas to read as though they were agents before you type FINAL on that manuscript.