It is hard to say, but If I had to guess off previous history:
The latest snapshot is 15w41b.
Which means the Mojang has entered its 41st week of developing 1.9.
The previous 1.8 update finished at 14w34d:
Which means Mojang was on its 34th week of developing.
...
Actually the more I read into it, the way the updates work is not very consistent because Mojang doesn't have a public deadline. I can't base it off of previous versions because there were so many major updates in 1.7 - 1.7.10 and then in 1.8 - 1.8.8 they have only fixed some security and glitching issues.
Since Mojang just changed Minecraft with the addition of Elytra, and and has added so much more in just the last month; they're far from finished. I would expect to see an update around Christmas time, or even next year.
