I can't really comment on when 5.7 is scheduled, nor do I believe Franz has given a timeframe yet, but:

- I got the impression from one of their Totally Random shows, that 5.6.1 is sort of a final release in terms of features for 5.6.x. That's just come out, and Andrew said he'll be closely monitoring bugs to be able to create a 5.6.1.1 release ASAP, which will be pushed live to the auto-updater. And then when this is done, the focus will be on 5.7.x development. I'd guess that he's working on things now
- From Franz's comments lately about the planned features and his enthusiasm for 5.7, I'm guessing we'll see previews of 5.7 earlier, rather than later. I'd wager that lots of the new features have been prototyped already.
- On the other hand, they also gave the impression they want to build it to a very high quality before releasing it properly, so I don't think we'll see anything in the next couple of months. My guess is sometime around the middle of this year. (just a guess though!)
- There is a 5.7.0 branch of concrete5 in github. I'm not sure if Andrew is pushing his development regularly to this branch, or he's keeping it private. But if you are very keen to see things as early as possible, this is where I reckon you'll be able to see the progress -https://github.com/concrete5/concrete5/tree/5.7.0...

(I could be completely wrong here, these are just thoughts from what I've read/heard)

Now about your project, I would say use 5.6.1 and start now. I'm confident that there will be a well tested upgrade path from 5.6->5.7.
You could at least do the theming, custom block development and things like custom dashboard pages without fear of them not being able to work on 5.7 (or at least with minimal bug fixing).

If I had a big job ahead, I'd be much more inclined to get a head start, even if I had to do some upgrades along the line, than wait for a new release and be 6 months behind!
Even just learning more about concrete5 and it's API is going to be beneficial for you in completing your project.

Thanks,

but fear not, I did already start, (I am not much in the waiting thing...).

Actually, the question was more like:

Do I start on 5.6.1 stable release, or Do I start from the 5.7.0 github branch straightaway since that will become the defacto standard at a point during this project development.

Use 5.6.1. - then you'll got an official upgrade path to 5.7.

Anything in github should be considered unstable and only for testing/investigating.

As Mesuva said.

There has been other discussion and questions about the state of github branches, so 'which github branch to use for 5.7 enthusiasts' would be a good question for Totally Random and may help to focus core team attention on clearing this up.