On the Laravel News site they've posted about the framework release schedule for the next few minor version releases and the support levels each will include.

At Laracon 2013 in Washington D.C., Taylor announced the first official release cycle for the framework. [...] By having an official release cycle, it allows us as end users to plan around when we need to perform upgrades and also the dev team a way of knowing what is coming and when. Since this announcement all new releases have followed this schedule.

He talks briefly about the 5.1 release of the framework and how it changed up the flow by adding long term support (two years for bugfixes, three for security). He then outlines the release schedule from the 5.1 LTS release out to 5.5, the next LTS release. This is only a guideline for now and could possibly change in the future but at least it gives an idea of the schedule of things to come.