Predicting the future is very difficult: We leave that job to the Future Perfect development team. It is possible, however, to make a decent call about when then next Subnautica development update is going to surface.
We've set a rough date of January 15th. We may deliver Update 8 earlier or later than this, but the update will come around that time. The biggest feature we expect to add around that date is save/load. If you've been playing in Experimental branch (See this guide to find out how to get it) you will have already been able to play around with Jonas' prototype.
Normally, we want to release a substantial update every two weeks or so. Update 7 appeared on December 23rd, which means we're coming due for another one soon. But it's the holiday season, and much of the team is taking a break at the moment. That means we are running a little slowly.
Other things that you might see pop up in Update 8 are more creatures, several prototype tools, a tweaked main menu, and an overhauled head-up-display / user-interface. Keep an eye on the Subnautica Trello Board to see how we're progressing!