Last week, DH and my two older kids got in line during MM for Space Mountain Ghost Galaxy about 5 minutes before the park opened at 9 a.m. for everyone else. The two younger kids and I were walking toward there at a more lesiurely pace.
By the time we reached the FP machines, a wave of people came from that direction (just got the FP right when it activated at 9 a.m.) and FP return times were already at about 10:40. While I was there getting our FPs for later, the crush of people getting in at 9 arrived to ride SMGG or get FPs. By 9:30 the FPs return times were into the afternoon.
SO, If you want the FP for early on in the day, be there right when the machines start handing them out, at least when it's a holiday overlay.
PHXscuba