Each box of supplies reduces the downtime at that very factory by 4 minutes (as you are actually told by a system message after each drop).
So the number of boxes to get damage undone depends on the current downtime left (factory buldings start at 180 minutes) and the number of players doing resupply runs at the same time.
Perhaps a more drawn out example?
Enemy bombers hammer your ammo factory down to %80 all in about a 30 second period. So THAT %20 will be down for roughly 180 minutes give or take 30 seconds. Now you want to help on the home front instead on the war front so you hop in your C47 cargo plane and haul field supplies to the ammo factory. Keep in mind that when a factory building is destroyed it has its own timer, and the default time is 180 minutes if nothing is player resupplied (trains are another subject). So once you arrive after your 5 minute flight and drop that box of field supplies you have just taken the repair time from 175 minutes to 171 minutes, AND you've earned bomber perk points!
You can see how if a squad of 10 players each upped a goon how it would cut the time of the repair down drastically from the default 180 minutes. That squad of 10 players can reduce the repair time by 40 minutes with each flight to the damaged factory. After four flights of 5 minutes each the factory should be back to %100 thanks to the 160 minutes that are repaired and the 20 minutes of default repair time that occurred.
The curve ball is when the factory is damaged in different intervals. For instance, if B24's hammer a ammo factory down to %70, then a set of Ju88's hammer the same factory down to %45 thirty minutes later, there is two different default timers at work on repairing the ammo factory. However, when resupplying that factory everything is repaired and each building has its repair timer reduced by 4 min. The building destroyed by the B24 will be repaired first and then buildings damaged by the Ju88's will be next. First damaged is first repaired, so to speak.
