Taken from stackoverflow main meta (link):
How many flags do I have?
When you start out you are alloted 10 flags per-day. This number may increase to up to 100 flags per-day:
- You get one bonus flag per 2000 reputation.
- You are awarded additional bonus flags when you flag correctly.
It would seem that - at low levels - one bonus per-day flag use is added any time a flag is deemed helpful. I don't know if there is some sort of dimishing return on the long run (ie: the more you are close to the 100 flag/day cap, the more helpful flag you need to increase the counter).
EDIT: based on some research, it would seems that the system still uses the old flag weight variable to calculate the available flag per day count. I can confirm that the flag weight indeed had a not-linear grow rate (cap is/was 740 if I'm not mistaken, but at about 500 it would start to grow at a less than linear rate). I suppose that it is still the same... maybe someone has the complete formula.