I'm not sure there is a practical upper limit. Ghostscript for example for years only released as open source the version prior to the current one, and no one claimed that the existence of a proprietary fork invalidated the status of the open source version, no matter how long the delay between releases.

Admittedly there are differences, in Signal's case the deployed version is not currently released at all, but the worst case would be "open source version is no longer maintained", at which point I suspect that several forks would spring up, causing some confusion until one gained traction and dominance.

Right now, the latest release is 9 months old. This is certainly annoying and even concerning, but not yet cause for outright alarm, which can probably wait until March has gone by with no announced release schedule.