How quickly does flexibility improve?

How quickly can I improve my flexibility?

I used to be able to do a full split 10 years ago, but now I can't. I can currently descend to 20-30 degrees.

How much time would it take me to reach the full split if I train flexibility 3 times a week?

How easy is to improve your flexibility? Does it work like muscles development? Do I need to take rests throughout the week?

If you were 12 or so when you first did the split, now it is a very different thing.

When you grow the pelvis tilt forward, muscle and ligaments became stiff.

If you werent a child, probably it is easier now considering you still have some sort of muscle memory.

You need to do passive (static and dynamic) everyday and active stretching every other day.

You can do it in 1-2 months since you are not untrained.

I was exactly 12.

What are some good passive and active stretching exercises?

If you train it everyday you should be able to get it within a month

Woah now I'm impressed and excited, going to stretch daily and report back.

I still would like stretching exercises suggestion though. I have been doing the ones from drillsandskills.com (split section).