The answer is, quite a while!
We've known for some time that Warner Bros had starter work on a sequel, but after the enormous success of The LEGO Movie I didn't think the wait would be so long.
May 26th 2017 is the proposed release date, over 3 years away! I can appreciate that this kind of film takes a heck of a lot of work, but I'm very surprised they are not getting this out sooner.
Warner Bros also have a Ninjago movie in production, there's still no word as yet if that is actually going to happen, it could be that they are solely concentrating on The LEGO Movie 2 right now. We will see.
If you've not see The LEGO Movie yet, grab yourself a kid and head to the multiplex this weekend, you will not be disappointed!