It depends on many things: How busy the park is, how many people are in the line, and how the line is handled.
Personally, I don't use most of the
Disneyland SR lines, even though I usually travel as a single. The reasoning is that many of Disneyland's singles lines are not run like those at most other parks. At Splash, Indy, and probably some others that I'm forgetting, Singles are merged in with
DAS parties for the first part of their wait. There is no reason for the lines to be shared, and it makes singles lines longer for no good reason. The line moves better once the lines separate, but to be in a wait made artificially longer by arbitrarily pairing two lines that have different functions and should be separate is infuriating.
Thus, the only lines I recommend are RSR and Space Mountain. Both are run properly and will dramatically shorten your wait. Perhaps more people know about it now, but in 2018 it was new and little known, and it basically made Space a walk-on. RSR's line is better known, but moves well, and you can generally get on in no more than 30 minutes, often less.