I would say if members stop starting discussions or suggestions ideas. If there aren’t many newcomers and the ones that do join only hang around for a few days before leaving.
Sometimes the members who are currently there don't want change. If you make too big of a switch on a forum, you might lose people who have been long time loyal members.
The first year is all about creating that first content and getting small community established on the forum. If a small community is established, then I move on to year 2 and do heavier marketing efforts to see if the community grows. If it grows, then I set an activity goal to be met by year 3.
I usually only cease to operate a forum if it hasn't budges after year 1.