How long did you take to get to sub-15 since the time you cracked sub-20?

[bran]

about 2-3 weeks

[yockee]

Well, I became fully sub 20, January of 2011. Right now, I average low 15's. I'm just about sub 15. I get 13 ao5's, and 14 ao12's.

[nightpegasus738]

about 2-3 weeks

That's fast!

[Forte]

I don't even remember this happening, but that's probably because I didn't consider sub15 as a thing at all.
According to WCA, it looks like a year, but I improve slowly so you can probably do it faster.