How long do people wait before doing new lessons?

I agree with the “below 100 apprentice” rule, but another thing that might be helpful is the Lesson Lock script (assuming you use scripts)

The default settings “lock” lessons at 100 apprentice, which lines up with the suggestions here. But I personally use the below settings for the scores so the score is the estimated number of daily reviews

I wouldn’t worry too much about your guru count rising unless you have a ton of leeches that keep moving back and forth between apprentice and guru