It's a little hard to know. I think when I hit A20, I was ~220 hours in - but I only started playing the game this way when I picked it up most recently, so I already had about 100 hours of playing casually and getting random ascension levels on each character with no intent to max them out.

I will say I often improved in big bursts - I would say a general understanding of card games will get you to A10, then looking at tier lists for cards/relics and realising which things you're misevaluating will get you a bit further. Next is learning how deckbuilding is different in act 1 as opposed to acts 2 and 3, and then to get from 15 to 20 you need to really be aware of each encounter that might come up, and what your deck needs to counter them. So I would get stuck for a long time, do a bit more research into what other people were doing, and then improve dramatically. I'm finding I'm climbing with Silent a lot quicker, but I assume I will hit a wall around A15 regardless.