PPFD is the measurement of how much light a plant uses per meter squared per second. It is what you look at when deciding light intensity. The problem with measuring PPFD is that the meters cost hundreds of dollars. A quick and relatively effective alternative people use is measuring lux and translating it into PPFD. There is no straightforward equation for PPFD to lux or the opposite because it changes depending on the colour of the light (the spectrum).
This is the best calculator we have right now:
Online calculator to convert PPFD (micromoles per second per meter squared) to illuminance (lux).
www.waveformlighting.com
If you find the spectrum of your light, you can select it and input how many PPFD you'd like. and it will output how many lux you'll need.
Now to your question, the intensity at which your light will burn your plants depends on a lot of things but some pretty easy numbers to go by are:
Seedling: 300 PPFD
Vegetative: 500 PPFD
Flower: Up to 1000 PPFD
A cool thing to think about is that up to 2500 PPFD which is an insane amount of light, when people have too high light intensity, it actually has nothing to do with the light. A cannabis plant can handle up to 2500 PPFD (even a seedling) if it has all the other conditions perfect and adjusted accordingly. The higher the light intensity, the more nutrients it needs, the more CO2 it needs, the more water it needs... So when people "burn their plants" They actually just have an imbalance in one one of the other parameters. Now, that doesn't mean that you should suddenly bump up your lighting to 2500 PPFD because above 1000 it becomes very difficult to give the plant the perfect amount of everything it needs.