The engine has to be running for it work.
In 16 months, and 28k I have still yet to notice a regen, and believe me I know what I'm looking for,(long story for another day but got rid of my T5 because of faulty ECU and regens). I do however do a lot go motorway miles, and on occasion notice a dramatic drop in mpg on a regular route where I know what to expect. I was told somewhere they will do a regen every 400 hundred miles on average, so that would be two a tank.