Major versions often include low priority bug fixes, and are released twice per year. The first is in spring, and the second happens in the fall.
Sometimes there are bug fixes that need to be released sooner rather than later. NetSuite keeps an updated schedule of eFix releases at the following location in SuiteAnswers: https://netsuite.custhelp.com/app/answers/detail/a_id/34424/
For a summary of updates to the help file, see KB 36156: https://netsuite.custhelp.com/app/answers/detail/a_id/36156