I've only gone during crowd seasons and I have found that the afternoons are usually better than the morning for that ride. I've gone by there 11am or so and found the line at least an hour. Later in the afternoon we head over there and it is around 40 min. I'm not willing to waste DL time to wait in line at DCA.
I know a lot of people say the line moves fast, but to me it feels like the slowest line in the whole DLR. It is boring and hot and there is nothing to look at. When I saw videos of the line at WDW, I was really disappointed that ours is so crummy.
I'm curious to see how Little Mermaid affects TSMM this summer (if at all). Will it cause the TSMM line to shorten because everyone is at the other ride? Or will it bring even more people in to the park? Last summer, with WOC opening, I was surprised how many people were in DCA. It was much more crowded than before.