The six-year restatement period applies only to pre-approved plans. Thus, it would apply equally to a governmental pre-approved plan as to a nongovernmental one, and the relevant dates would be the same. Individually designed plans are required to be updated each year, but are not entitled to receive determination letters at all except upon adoption, termination, or certain corporate transactions.
However, for a variety of reasons, governmental defined contribution plans are far less likely than private plans to be pre-approved plans:
Many governmental plans are adopted on a statewide basis, and cover all employees in a particular job category (e.g., teachers, judges, legislators) within that state. Because they are larger than most private plans, they often have access to the kind of legal expertise that enables them to have individually designed plans.
State and local governments, other than certain grandfathered ones, cannot legally adopt 401(k) plans, which are the most common type of pre-approved plans.
The same pre-approved plan document cannot be used by both governmental and nongovernmental plans. Rev. Proc. 2017-41, Section 9.06. Thus, many pre-approved plan sponsors simply don't offer pre-approved plans to governmental employers.
Governmental plans are subject to state law, and of course there are 50 different state laws, so it is harder to have documents that will work for all of them than it is to have documents that will work for ERISA-covered plans.
To the extent that a governmental plan is not a pre-approved plan, the six-year cycle does not apply to it.