Simply stopping the DHT isn't enough. After the cells have been exposed to it for so long they just flat out stop proliferating. You have to find a way to signal the cells to start proliferating again.
On another note, I have hair pictures going back to 2012 which is when my hair loss got really bad and I first experimented with minoxidil. 2010 is when I first started losing hair. My hair hit its worst point in March of this year. Right now I'm about where I was in late 2013, early 2014. So that's right at your 30 month window.
If I could get back to where I was in mid 2011 most people won't even notice I'm balding. I need to get back to some point in 2010 before I'll be happy. The last time I know my hair was perfect was March of 2010. Unfortunately for me, that's more than double the window. From mid 2011 to late 2013 is when I lost the most hair. From early 2013 to this year was a slow loss. As such, even though I've gotten back 30 months worth of hair loss, it's not that big of a difference. It's the 30 months prior to that which took me from NW1 to NW3. Since then I went from NW3 to NW4, and then back to NW3. Based on the vellus hairs that I have growing in, it looks like I can get back to where I was in 9/2012, which was NW2. That's the oldest close up I have of my hairline, which is the point where it got so bad I first tried minoxidil. I wish I had stuck with it then instead of quitting after a month or two. I don't think I will be able to improve beyond that without a hair transplant.