It's updated as often as Atlassian releases new stuff it needs to support. This can be as frequently as 2 weeks.
However, they do not remove anything from it without making it a completely new release. If you were using calls against the version 2 REST API from the day it arrived, then those calls will continue to work the same until the version 2 REST API is retired and turned off. They may have new functions added to them, but they won't have anything removed.
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