Steam claims 12 hours, but I know several of those were working on bug reports and simply being afk. Still, I've spent quite a bit of time on it as well and am very much enjoying it. There are some rough edges, and even a few sharp ones. I'm amazed to think how soon the game is coming out, but I think it is much more pulling back the curtain than adding entirely new things at this point, and Chris is some sort of android with how much work he gets done (and the other devs too!)
I'm particularly looking forward to the next patch because there is currently one rather critical bug that stops most games from being winnable via lack of access to requisite buildings.