An Open Source project that does not update its web site, has not done any commits in the source code repositories or modified other resources within a given time period will appear dead to the rest of the world.
A project that is not updated, is in this state because the people in the project do not care enough or do not have the time or energy required to keep the project afloat. If people do not manage to care enough to keep things up-to-date, the project will be considered dead. Yes sure there are exceptions, but not many. We all look at other projects like this, we must realize that others look onto our projects the same way.
There is of course no exact number or scientific answer here to what exact time interval a project needs to update with, but a living project does not have to think about that.