I would have said a console generation before but honestly, game development for some seems to have slowed to a crawl that they take a whole generation (or longer) before we ever get a new game in a series. Grand Theft Auto comes to mind off the top of my head. Dragon Age hasn't had a new mainline game in a decade, for example. But we know both series are getting a new game so they aren't dormant. Although, by the definition of dormant we could call Dragon Age dormant; it'll just become active again once Dreadwolf finally releases

Over 10 years without a game makes a franchise dormant to me.

Kid Icarus, Punch Out, Wario Land, Syphon Filter, Project Gotham Racing, etc. are dormant franchises.

You know Dino Crisis? .....yea like that lol

I consider a franchise dormant when Microsoft buys a studio. Seem to kill a lot of IPs lol

Grand Theft Auto's problem is that it's a seemingly bottomless cash cow. Rockstar could shut down everything but GTA Online and they'd still be battling EA for first place among third-party companies. Dragon Age would be getting more regular releases if BioWare wasn't ailing as a studio. Anthem wasn't a great idea, and Andromeda killed their reputation. 

When the publisher and/or developers stop working on it.

Not the game you had in mind, but this exists:

I consider a franchise dormant when the fanbase abandons the franchise.

Once upon a time I think I would've said missing just one console generation is all it takes, but with the increase in development times nowadays I think that expectation might have to be adjusted. Maybe ten years? Generation and a half, roughly?