3 months for a 1.090 belgian dark strong, and even then it took another 6 to be drinkable. Stouts and porters have taken me between 6 and 8 weeks to carb up.
Grav and temp are the two biggest factors in time it takes. And just because it is carbed won't mean that it still isn't green, and needs more time to condition.
The
3 weeks at 70 degrees, that that we recommend is the
minimum time it takes for average gravity beers to carbonate and condition. Higher grav beers take longer.
Everything you need to know about carbing and conditioning, can be found here
Of Patience and Bottle Conditioning. With emphasis on the word,
"patience." 