I found with the first several PLLs I learned that as I learned a new one I would "forget" some algorithm I already knew. I finally came up with a way to learn algorithms that addressed this problem for me. First, I tackled the PLLs two at a time... for the most part, two a week.
Once I got one new algorithm into my muscle memory, I would start timing myself like this: Start timer and with a solved cube repeat the algorithm however many times necessary to return to a solved state then stop the timer. To help with recognition, I would try to visualize the cube scrambled for that alg. as I was timing myself.
I would do a couple of average of 12s and then learn the second algorithm. Once it was in my muscle memory I'd do an average of 12 as I just described. Then I'd do a couple average of 12s for the first alg, then a couple for the second alg. Then do average of 12s on algorithms I learned in previous weeks. Then I'd do some solves for a while before doing more average of 12s on the two new algorithms. Then more solves, then more average of 12s on the new algorithms, and I would repeat this as many times as possible over the course of the week.
I kept a log of my averages of 12... it was always nice at the end of the week to see how the times dropped.