I do my dry-hopping for 7 days tops (always with pellets). I get a little bit of hop sludge in the bottle, but haven't run into any grassy flavors. I wouldn't push it any longer than 2 weeks though.
The more important thing is to verify your fermentation is really done before you bottle. When you transferred, was it just the end of visible fermentation (no more airlock activity) or did you take hydrometer readings to make sure your gravity had leveled out?