Well, I first started training it as a roundoff backhandspring and gradually rasing it up. After about 4 months, I hadn't really gotten anywhere with it, so I gave up.
I decided to have another go at it more recently, this time training it as a roundoff layout. It took me about half an hour to get comfortable doing timers over the table onto my back, and took me two weeks after that to get (reasonably) comfortable doing timers to my back over a table at competition height.
This was only within the last month or two, and I have not yet actually flipped it.
Keep in mind, however, that I was already (all traces of humility set aside) a very powerful tumbler with very strong aerial awareness before I ever even started working yurchenkos.