At what volume levels do you see improvement?

I started swimming 2 years ago as part of doing triathlon (please, no one beat me up). Up to this point while I've taken swimming somewhat serious I've certainly let biking and running be my focus while just trying to be an "okay" swimmer. Currently, I swim 10@100 at around 1:25 ish (I don't leave at 1:25 by my actual swim time is 1:25 per 100) with a god-awful flip turn. My volume starting this season is roughly 8 - 10k/week. I really want to start putting more emphasis on swimming and becoming a better swimmer. I enjoy swimming and think there is a lot ahead of me. I met with a good swimmer and he is starting to point me in the right direction. The first thing we are doing is just upping my volume. I'm going to start putting in 12 - 15k each week. My question for the good swimmers is: are there volume levels where you noticed improvement in your swimming. I mean, for example, when you starting putting in X yards per week consistently you noticed an improvement in swimming. I know this is probably different for everyone but I just wanted to get a general feel... I know I can translate this into running - when I hit 50 miles a week I can feel the difference in my running. And from there it is usually in 10's for me. 60 miles a week and 70 miles a week. Thanks.