Does anyone know when demo 2.7 will be released? 
Never. The next one will be 3.0. 
There are already some betas of the new engine released in the demo topic, if you want to try the latest out. Not as far in the Frigate as 2.6, but it uses a new engine with better special effects and other such things, and I believe DF said the next non-beta release will go just as far as 2.6 went.