Hello,
If you check all the releases dates (from the article given below), there have been a gap of almost 6- 9 months between major releases whereas minor releases are more frequent between like 2-3 months.
The Latest version of Symantec Endpoint Protection 12.1.RU6MP4 was released on March'2016.
So I believe you can expect a major release in the coming months for sure. (atleast that's my personal opinion)
Check this Article from Symantec:
Released versions of Symantec Endpoint Protection
https://support.symantec.com/en_US/article.TECH154475.html
Regards,