What percentage of flowering is the stretch phase?

I know it varies a lot from different strains but for the typical indica/hybrid would you say the main stretch period is more or less than half of flowering time.

I have a hybrid 3 weeks into flower and it's starting to really fill in my scrog screen. My first scrog so I want to know if I overgrew the plant or not.

As you noted, it varies but you should be through the stretch very soon. Another week tops. Push the main colas to the outside of the screen. If you run out of space just let them grow up at the edges. That way they won't cast a shadow on the rest of the plants.