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.