How often do you have players do investigator improvement?

In the official rules, it's after an adventure is over, or for longer campaigns, I've heard there is a kind of milestone system to check at the end of chapters. Just curious how many people follow official rules vs homebrew improvement, and how often you have investigators make improvement rolls for skills?

Hi similarly, I usually do it at the end of an adventure, when the characters are having downtime and recovering. When I ran the Horror on the Orient Express, we did it each time they rested on the train.

somewhere between session and milestone

I'm running Horror on the Orient Express and do investigator development after every session. Sure, some players have high skills but they can still fail, you just got to ask for Hard or Extreme rolls instead. Also the higher their skills get the less chance of it getting raise, so I think it balances itself out. I've also given the players free points at times to just add or told them to use penalty dice when doing an improvement.