The timing of dry hopping is debated regularly here so you'll get lots of different responses, none of them right or wrong. Meaning dry hopping is pretty subjective. Some people like to dry hop as fermentation winds down, some in secondary as the beer gets more clear, others dry hop in the keg. At room temp/fermentation temp, you'll extract the dry hop goodness in 3-5 days IMO, with a longer extraction period if the beer is colder. I'll tell you what I do - not saying it's better or worse than other methods, it's what I like best. After fermentation I give the beer sufficient time in primary to drop the vast majority of yeast out of suspension. Yeast absorb hop oils, so beers dry hopped with a lot of yeast in suspension tend to have a drop off in aroma as the yeast drop out. I feel that aromas last longer when the beer is dry hopped when fairly clear. I rack the beer into a purged keg with dry hops (contained in a fine mesh nylon bag) in the bottom, wait 3 days @ room temp with head pressure on the keg, then chill and finish carbing.
The practice of dry hopping is really worth experimenting to see what you like best. Pretty much all methods make good beer.