According to this quick post on the Laravel News site the next version of the framework that will get long term support (LTS) will be version 5.5:

Version 5.1 was Laravel’s first LTS release and its two-year window of bug fixes are coming to an end this year. There have been a few people questioning if another LTS would be released and version 5.5 would be the next in line if it did happen.

Just today Laravel announced on Twitter that Laravel 5.5 will, in fact, continue the LTS line. [...] Just as the previous LTS this will include two years of bug fixes and three years of security updates.

Long term support means that the version will be "feature locked" on release but will continue to get bugfixes for issues found until the end of the maintenance window is reached.

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.