FWIW, I'm using 29% H2O2. I'm still fine tuning the dosage, but it seems to be working so far...
I'm using 10cc's (mL) per 3L (1 1/2qt) of water. I never saw them until I changed soil brands when I couldn't get the normal stuff.
The flies themselves don't appear to be doing any harm on my plants, but from what I've read of the life cycle, the period for eggs to hatch is shorter than the time it takes for my pots to dry out. I have to either keep the soil moist (with the H2O2 solution) or use a pesticide (last resort) in order to be sure that the eggs don't get a chance to hatch.
That said, I'm no expert - I'm still getting a handle on this stuff. We'll see how it goes.