Estimates not available, not even in Valve Time.
Fan: Is Valve potentially already working on a new engine?
Fan: Source 2? Could or… could not be?
Gabe Newell: We’ve been working on Valve’s new engine stuff for a while, we’re probably just [incomprehensible, subtitled as: "waiting for a game to roll it out with"]
Fan: Is it going to be more than just an extension to Source? Is it an entirely new engine?
Gabe Newell: Yeah!
Seriously though, a new overhauled Source engine would really be something. Rumours started stirring about this potential development months ago, but this is a step towards confirmation.