We'll keep PHP 5.6 around as long as it is available in a reputable package repository that is compatible with the server OS.
When the time comes to end support for PHP 5.6 we'll give plenty of notice to give you time to prepare.
I don't know when that will be, but it will happen at some point. Upstream support for PHP 5 officially ended almost 3 years ago, so if you haven't started preparing already then you should start thinking about it now.