I don't understand the talk about botting time limit, surely people with outside jobs bot at least 8 hours every weekday, at least niantic must assume so. Golfcourse caretakers driving around all day cutting grass, never standing still, and surely some having the bracelet that makes them keep catching and hatching during the days all days. Taxi drivers having the app on all day, also catching when having some free time, hatching eggs. Speed shouldn't matter since niantic doesn't know our lives. Time shouldn't matter unless it's on 24/7, only very high xp/day or something I must imagine is the cause...or teleporting.
The thing is any ban theory is pure speculation (except API mistakes). Niantic does not release their rules of flagging, so all people could do is to speculate --- and all sorts of theories (but in fact are all hypotheses) pop out.
How much one plays, how long each playing session lasts, and how high the account lvls are, of course could all be factors. However, it is unwise to claim accounts are banned because they are over lv 30. Account lv's are high because they have/had been played for quite sometime at this point of time. For example, my account was already lvl 31 before I ever started to bot and spoof on it, and that was way back in last November. Therefore, it is ridiculous to propose simply being high lv's would be targets. Additionally, it was not uncommon for me to play the real app for more than 12 hours in a day legitimately at the time.
A great portion of legit players who started playing last August or so would have been lvl 30+ by now, 35+ if hardcore, so don't get the cause and result reversed - high lv accounts are banned not because they are high lv's, it is because many, if not the majority, of accounts that were created more than 3 months ago are high lvs by now.
If you have botted your accounts for more than two months and still not lv 30+, you might as well play legitimately and receive the peace of mind as a bonus.
How fast one reaches lv 30 (exp per hour/ catches per hour / travel distance per hour, etc.) and how one plays each day (any breaks? short or long? how many short ones and how many long ones? etc.) by far would be more important factors than the lv's and total playing time per day --- in my opinion and pure speculation, of course.