I haven't yet had bottom surgery but since I will have it in a few months or so, I think I can at least give an approximate answer.
I had my top surgery pretty much exactly 4 years ago, in 2014. So, the time between my top and bottom surgeries will be roughly a few months over 4 years.
My reasons for taking 4 extra years between, as I was offered consultation for lower op already 6 months post top op but declined it at the time:
Back in 2014 I also had bottom dysphoria like I do now but it was more mild-moderate then, and even though it had just kept getting worse all the time already at that point, I refused to let go without a fight. I wanted to know I had tried everything else first. I kept trying to make my parts work for me somehow, but dysphoria just kept getting worse. Finally I hit my breaking point where I couldn't keep fighting it anymore and dysphoria sky-rocketed, and ultimately realised I needed bottom surgery. So I started planning and researching for that.
Ever since the beginning of my transition I knew that if I was gonna ever have bottom surgery it would be meta. I looked into both meta and phallo, but it was meta that I ultimately decided for, for all sorts of reasons. So that's what I'm gonna have, but the journey to get to this point was quite long and crooked and I'm still not quite there yet. All in all I had considered about bottom surgery on and off for around 6 years before I had made up my mind about it, as my transition started in 2009. It always felt like a huge step for me, much more so than top surgery or testosterone. That made me also more careful with my decision, and meant I needed more time to be sure.