Wontolla has made a good point. Depending on what you want to get out of learning your PLLs should be what decides your time frame. Most people just suck it up and learn all 21 in less than a month (1 PLL a day is certainly do-able, and learning all of them in a week (which is less than 3 a day since you already know at least two from knowing 4LLL) is also do-able) and learn to recognize the cases by brute force solving. If you'd rather have a great understanding of each PLL (recognition wise) before learning a new one, I'd say this is the wrong way to go about it (because it takes a good bit of time to learn each PLL from two sides and have less than 0.5s pause for recognition, for me anyhow).
If your intentions are to learn all PLLs to reduce your LL times, I'd learn at least two similar PLLs a day. What I mean by this is learn the mirrors (such as J's, R's, Ga/c, Gb/d, etc), then learn the single ones. T is always easy to learn though and really fast, and you can learn an F perm that is just a conjugate T perm. Or you could learn V and F in the same day as they are only two moves different if you use the algorithm I have in mind (the V alg that starts with R' U R' U' would be R' U2 R' U' for F, then there's an extra U' in the V alg if you're doing the F PLL).
Honestly learning PLL recognition will take time, and you are better off (in my opinion) learning all 21 and having a moderate recognition and recall, than spending a lot of time on each alg. If you could learn a new PLL in a week, and your recognition is as good as knowing what case you have from looking at any two adjacent sides, then it would still take you 21 weeks (5 months). In my opinion that is far too long and you'd be much better off just learning one PLL a day then reading a good PLL recognition guide (stachu's or sarah's) to learn them from two sides.
Don't forget, you can always do PLL only practice, where you concentrate on getting each of your PLL's sub2, or sub1.5 for the easy ones (but set a goal for sub3 and sub2 for easy ones first, because getting a fast PLL does take practice).
Also, when learning your PLLs, keep recall tricks in mind. Such as the Ra vs Rb algs. The Rb alg starts as R' U2, and Ra starts as R U2. The easy way to remember this is which way your block is pointing, move R accordingly. Rb's block points to F, so move R in the same direction, whereas Ra it points to B, so move R towards B. You can do this for the G perms (block on R) as well. Just look for those types of tricks, and that should help your recall out a decent bit (until the algs set into muscle memory).
Another thing: Don't forget that you can benefit from some OLLs when learning PLLs. Y and E perms are a great example of this. The common Y perm is just two OLLs (kite + T, the SM + SH T OLL) put together, and the E perm (RUD one, anyhow) is also two OLLs (chameleon and bowtie).
