What is too much of anything? Mostly this is unknown. Certainly it is unknown for most forms of electromagnetic energy and the effects on biological processes. There are some guidelines, for example ICNIRP:
https://www.icnirp.org/en/activities/news/news-article/rf-data-gaps-statement.html
The ICNIRP is the International Commission on Non-Ionizing Radiation Protection. They will give you graphs and calculations to help you attempt to answer this question for specific environments and exposures, if you can get accurate technical specifications from each PEMF manufacturer (good luck with that).
Even this highly technical society that specializes only in one narrow range of electromagnetism effects on biology will start right off by telling you right up front what the huge knowledge gaps are.
Basically, you could study this full time for your whole life, and not be able to answer your question.
So, what can we do? Well, we can use common sense. Here is the logical guideline that I used when developing ICES-PEMF technology:
What is the smallest fraction of an electromagnetic pulse that can be used to result in the most desirable biological effects?
It turns out that after about 20 years of studying this, I was able to reduce the energy contained in magnetic pulses down to about 1/500th of a full “square wave” while still retaining all of the measurable biological benefits.
This means that I could get the same biological benefit from 300 mW (0.3 Watts) of power as crude PEMF systems could get from 150 Watts.
So, when using ICES-PEMF you are getting exposed to only 0.2% of the electromagnetic radiation power that you would get exposed to when using a crude PEMF system. So, at the simplest level, if a safe exposure time for a crude PEMF system is 1 hour, then the same dose from ICES-PEMF would be delivered over a 500 hour period. Or, conversely, if ICES-PEMF should be limited to 1 hour for safety concerns, then crude PEMF systems should be limited to 1/500 = 7.2 seconds of total use.
Since no one has done the trillions of dollars and centuries of scientific research it would take to map out all types of electromagnetic exposure at all power levels on all organisms for all biological effects, we need to make a few common sense generalizations. Here is mine:
Get the most beneficial effects you can for the least amount of total electromagnetic power in the least dangerous (lowest frequency) range.
Then, with just a couple of decades of work, the result is ICES-PEMF technology.
How much is too much? Depends, but in my opinion a good rule of thumb would be that it is about 500 times less dangerous than a functionally equivalent dose of crude PEMF exposure.