It can take a lot of time and neither I nor jaguar (I think) has ever said anything other than you need to reduce your volume significantly and take it very slowly.
Why reduce volume ? This appears to be another example of the braindead binary thinking you guys display. Why not instead go from doing 0% of your total training volume in flats (or whatever the starting point is), to 10%, to 20%, to 30% until you're at 100% ? That way one can take it slowly without taking time off training. That or start by doing speed work in flats, so that you build up a base of slow running in flats from your warmup/cooldown, and then introduce them into training runs, until you're using them for all training runs ?
Seems to me that your absolutist mentality is preventing you from approaching this problem (how to transition to flats) intelligently.