I had a similar situation not long ago with an IPA that I am currently drinkng. My carboy was full with a beer that wasn't ready to keg so I dry hopped in the primary 3 or 4 days before transferring to the carboy. I don't remember exactly how long it was, but I made sure the fermenting was done before the dry hop and it turned out great. The aroma is wonderful and the beer is very drinkable. Don't know if what I did was right, but it worked. Hope this helps.