For me it was around 4 to 4 and a 1/2 months. And don't worry if you don't improve as fast after being sub-25, it becomes difficult to improve after sub-25 IMO.
In my experience there have been three speed blocks. All you have to do is learn basic recognition and use a decent method.
Getting to sub-25 was easy. It was mostly just learning algorithms and figuring out how to solve the different pairs efficiently. I stayed around the sub-25 range for about 2 months.
After that I suddenly hit another spurt and knocked my times down to the sub-18 range. At this point I temporarily quit cubing due to complications that arose in my life.
After about a year of absence I've finally started back. It felt so horrible because I was used to getting 15s and 16s on average and now it was more like 18s, 19s, and 20s. It took a few weeks of casual practice to get back to my old sub-18 average.
That's why I hit another short roadblock. I managed to work it down to sub-17 but that was about it. I just couldn't find a way to improve. I stayed like that for about two months.
Then I suddenly hit another spurt and within two days I went from a 16 second average to around 13 seconds. A 12 second solve used to be extremely rare but it was now happening frequently. 10 second solves were almost non-existent but I was now managing them several times in an hour session. I've also managed several sub-10s. It feels super amazing! I'm so used to being slower that I still haven't gotten used to my new speed. I feel like I suddenly gained super powers or something!

Then of course I go watch a Feliks solve and feel slow again.
I have no idea what'll happen next. It might just be a slow grind from here on out but I suspect not. I still haven't mastered the ability to solve the cross and first pair in one sequence. I also have to solve the cross slowly if I want to preserve a pair or use keyhole. I suspect that with enough practice and study my understanding of cross+first pair solutions will suddenly click and I'll chop a second or two off my average.
For any of you sub-10 solvers, how was it for you?
I don't get how people are sub-20 after like 6 months, it took me 2 years
How long did it take you to learn full OLL and PLL?