Here is a post from someone who recently tried to RD MFSR but only arrived 30 min prior to opening. She exited 90 minutes after park opening (so that includes walking, waiting, and riding MFSR). It was January 7th which was a CL 7 day:
If you arrive earlier, you might have a shorter wait. In my experience, the wait for MFSR seems to go pretty fast even when it is long.