Forever lol
I mean… I like to think my directing is pretty advanced, but then I’ll read someone else’s story (e.g The Wall by J Miley) and go “OMG HOW” 
. So I don’t think you ever stop learning.
I think that when you first start out, that’s the biggest learning curve. The difference between my first story to second is leaps and bounds (but when I wrote my second, adding your own overlays wasn’t even a thing) and old me would look at my newer stuff with her mouth open going “how did you do that???” lol.
I find it much quicker to pick new things up now than I did at the beginning. Like, it took me ages to get comfortable with zooms and spot directing, but I picked up animated overlays and tappables a lot quicker. So the longer you’ve been around, the easier and faster it gets to learn new stuff.