The first preview builds have just been posted. Still have to wait for M to be announced with their final name by Q3 2015. Then assuming just like lollipop, all OEMS will start building upon M and adding their UI like touch wiz, a couple betas of Samsung testing, and then it'll be sent to carriers to do their testing, once thats complete, release.
My guess, and I'm pretty confident about my guess looking back at lollipop and how there were leaks of lollipop for the S5, and all the testing being done, I can safely say the GS6 won't receive M until Q1 2016.
Sent from my SM-G920T using Tapatalk