You don't really need to "learn" much with Petrus. It's just do 1/2 of F2L(2x2x3), EO(You have to learn this, but it's pretty simple), finish F2L(except only use R and U), then do normal 4lll, except you get to skip doing the cross.
Took me around a month. I’d only just cone back from a 6 month break since learning in 2018, and I wasn’t really concerned with learning fast. Also, I was bamfoozled. I didn't realise that the forums existed at the time, so I had to figure out everything on my own.
Lars guide was really confusing-at first I tried to do everything the classic blockbuilding way, but I gave up and eventually adopted a keyhole style approach to 2x2x3.
The concept of bad edges took forever for me to understand, and trying to blockbuild with only two sides was really restrictive for me, which added a couple of weeks to the process. Then I tried to do Lars LL system, then reverted to the beginner’s 3LLL approach.