The official release calendar can be seen here:
https://docs.moodle.org/dev/Releases#General_release_calendar
LTS releases have been following a pattern so It is likely that the next LTS might be 3.9 - (release planned May 2020) but HQ reserves the ability to change which release will be an LTS and it could even be 3.8 or 4.0 depending on the features under development at the time.
General help
LTS Release
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