Thing is, I have learned full PLL, and its ok to learn all the algs and comitt them to memory over a couple of weeks...But the thing is once you learn all 21 PLLs, after a few days, only some of them remain in your mind. Most of them eg about 17 of them, you will be unsure which one it was, and which one to use, so you need a constant cheat sheet for several months.
On top of that you start to get parts of one alg used with parts of another...and until this stops happening, you have not learned them properly.
Then there are days where you simply cannot remember the alg even when prompted and you have to go through it again several times and feel the muscle memory come back.
So unless you are doing the cube all day long, as i did when i was at school, and at nights...it is going to take several months of intermittent and quite regular practice to sink in full PLL,
If you are a youngster doing it on the way to school, at school, on the way home, all night at home, and in bed ..7 days a week, with young obsession...then i would say it would take you around 4 to 6 weeks to master full pll.
if you are working and playing with your cube sometimes at night, sometimes not, maybe a few minutes over the weekend here and there..then i say it will take around 3 -4 months to master full PLL