Some people consider flowers that contain more than 15% THC to be high in THC. However, for an experienced cannabis user, 20% THC may not produce many intoxicating effects, while only 10% THC could cause extreme poisoning in a novice user. In general, products with a higher percentage of THC will produce a stronger and more powerful effect. The cannabis flower usually contains between 15 and 25% THC.
The biological limit of the percentage of THC is 35%, so it is possible to find flowers above 25% of THC, although it is rare. Anything above 25% can be expected to be extremely powerful. It's rare to see THC levels above 25%. However, it is much more common to see claims of THC levels of 25 to 30% in marijuana.
Generally, the best quality cannabis you'll see is in the 25 to 30% THC range. In the real world, any cannabis strain with more than 20% THC should be considered very strong. In general, it's a good idea to expect that outlandish claims about THC are backed up by laboratory data, but most of the time this isn't the case. The percentage of THC in marijuana ranges from 10 to 30%, which means that the average percentage of THC is around 20%.
If you walk into a dispensary, you'll likely find this to be true when reviewing the menu. Ten percent is considered quite low for the THC content, but can be offset by a higher percentage of CBD.