Probably not much or any at all. How strong was the light leaking in? I used to on occasion check my flowering plants at night with a cheap 9 bulb LED flashlight and havent had any problems.
Seedlings should be getting lots of light... Light leeks during the dark phase aren't really harmful until you start flowering...
light leaks revert flowering plants back to a vegetative state... your already in veg, there is nothing to revert back too.
Flashing on and off of lights can cause hermies, depends on what kind of exposure and such. You're probably fine, but leave the dark alone from now on.