I think I can answer that question ("Is it the same for you?") at the end of the editorial in the affirmative, because once I've done the initial learning of something I need to play with it and then use it for real for a bit before I really know them. But once I really know them I wont forget them unless I stop using them for a while (and even then I will pick them up again quickly if I need them).
But there is a difference there: the play part of the process; to me that's an essential part of learning. For example I read up on the new windowed functions/aggregates, and then used them to look at how smallish prime numbers have some interesting properties (I'm a mathematician by training and for enjoyment, so that sort of thing is play for me); before I played about with them like that, I had some theoretical knowledge of them - I'd spent a little time reading the specs and translating the definitions into a set of operators on vector spaces over domains involving SQL types - but there was no imaginable way I could have considered using them for anything real. But after playing around for a bit, I began to feel confident, and I used them in real work [voluntary, not paid, I guess I'm retired now unless someone wants a JOAT (of course I claim that that's that's short for JOATAMOA not for JOATAMON :-D) like me on part-of-year and part time terms] a couple of times and it was easy.
I find though that even after I've learnt and played and used the stuff in real work I will still forget it fairly quickly when I'm not using it. I'd probably fail a Jeff M interview if I hadn't used GETDATE any time in the last three months if he didn't treat SYSUTCDATETIME as an acceptable alternative. But when I start using things again it all comes back quickly - look up the syntax once and it's back in memory - even my fingers have the muscle memory neeed to type it quickly.
But sometimes I have another way of learning too: when people say something is impossible that I think ought to be possible I sometimes learn rather rapidly, just to annoy them.
The other question - in the title and in the middle of the text is not difficult. There is certainly a limit to how much we can learn in a month or in a year. The science of physics indicates a limit - we can only change so many connections in our brain in so much time. For me, and I expect for everyone else too, there's a far lower effective limit than that. One can only go on for so long learning stuff without playing with it or applying it, if one exceeds that time some of the stuff learnt will just vanish.