NGINX F/OSS is updated approximately monthly, on a time-based release program. If you receive NGINX updates from a third party such as an operating system vendor, they may not issue releases as frequently and some vendors track the stable (i.e. legacy) branch rather than mainline (i.e. current) branch. Updates are announced to the email@example.com mailing list.
NGINX Plus is released approximately quarterly; check the NGINX Plus release details. Updates are sent to all registered users of NGINX Plus.
For details on the software release process, please check out the article NGINX 1.6 and 1.7 released.