Single rider for Rock n' Roller coaster

[Davey Jones II]

I know, from past experience, that this single rider line is usually pretty useless, and sometimes even longer than the standby line. However, toward the end of the night, it has been great for us, and got us in the ride much faster than standby. I was wondering at what time the single rider becomes useful; an hour before closing? Any guidance would be appreciated, thanks!
Also, how is the availability of LLs for this ride, on the same day? Our Tier One is Slinky Dog, in the morning; after that, is it fairly easy to get RnR and MMRR?