I'm a frequent solo traveler, so I'm fairly versed in the SRL. And, now that our youngest DD is old/mature enough, we do SRL on family trips as well. Here's my general experience/breakdown of waiting....
DL:
Indiana Jones: Generally the wait is 10-15 minutes; though it is constantly moving, which makes it a bit easier in my opinion. SRs will likely end up in the same vehicle, or one right after another, so the wait for the other SR isn't bad (1-2 minutes).
Splash Mountain: This doesn't move as fast as some of the others, but the line is typically short because it's a bit convoluted to find. Make sure you don't get in the handicap line (which is what many people assume is also SRL). If you use SRL early in the morning, you can get a few rides on Splash one right after another. Unlikely you will end up in the same log, but, as with other SRLs, you should be within a vehicle or two.
Matterhorn: Two SRLs here: one Fantasyland, one Tomorrowland. Typically they are somewhere around 5-10 minutes depending on the side. It's the farthest line towards the tracks. If you can see people up in that line, it will be about a 10 minute wait in my experience. You'll usually only wait for your other SR for 1-2 ride vehicles.
DCA:
Goofy: This almost always has a short line. SRL isn't always going on it, and I don't feel like it really gets me on the ride very much quicker.
Screamin': For this one, I've never waited more than 10 minutes. However, it's a popular one, so you may wait a few minutes to be allowed to go through the line. They will pause you at the loading platform, then send a group of around 5 people up and over the tracks (via elevator), and then down to the loading area, where they will dispatch you. But again, it generally moves pretty quick. Most likely you will have to maybe wait 1-2 trains for your other SR, but often they will be in the same train.
Grizzly: This is a fantastic secret. Holy cow, on a warm day, I'm so glad more people don't use the SRLs! On this one, we often end up on the same raft. Unfortunate part is that as a SR, you will almost always get an outer seat, which increases the potential soaking.
Radiator Springs Racers: This is always the longest SRL for a few reasons. First and foremost, it is well-labeled, so it's really obvious where to go. Other ones aren't so clear. Second, it's still a "new" ride. Yes, there's rides that are newer, but, until Star Wars Land opens, RSR is still the newest e-ticket ride. Third, the line appears kind of deceptive. You can't see that there's 20-30-40 people in front of you in SR. All that being said, there's a few ways to work around this. The earlier you get in SRL, the better. As with Splash, if you do SRL early with this, you can manage 2-3 times in a half hour or so. But after that, it can be a long wait. I tend to follow Fresh Baked Disney on Youtube. Their rule is "if you can see the end of the SRL at the bridge, it's a 45 minute wait." And, somehow, I always am behind the group of 2-3 teenage girls that go through the SRL to request sitting together when they get up to the front. Then the CM "punishes" them by making them wait... which makes all of us behind them wait too.