I make several RTs a year Annapolis-KP, since 2003. Excellent posts above. I have made the trip in 4 hours (ah, a glorious day and I must have been dehydrated) and as many as 10. The norm for me is between 4.5 and 5 hours, allowing for a fast pit stop on the NJTP for the cheaper gas and bio-break. All it takes is a breakdown on any one of the bottlenecks, or even Delaware doing one of its endless roadwork projects on its tiny stretch of I-95, to add hours to the trip.
EZ pass saves significant amounts of time.
I try not to arrive in the Metro NYC area anytime after 3 during the work week.
I listen to 880 AM for traffic updates. I usually take Exit 10, Outerbridge Crossing, or Exit 13, Goethals, to Staten Island, then Belt to Cross Island and on up to Northern Blvd. I did the BQE after the VN Bridge. Once. That was enough, and I have driven in Paris, Rome, Tangiers and other fun driving cities. Last resort is the GW Bridge and the Cross-Bronx, and things have to be totally stopped on the Staten Island route. I have done the tunnel into the city and the LIE, coming and going, but either very, very late or very, very early. I sponsored a USNA mid from Riverhead, LI, and he favored the Staten Island/Belt route as the most reliable.
Occasionally, if I don't want to deal with freeway driving, I will take the Eastern Shore route, going either way. Southbound, that's taking US 13 south or US 1 south (toll) after going over the Wilmington, DE, bridge and eventually finding US 301 to US50W and over the Bay Bridge to Annapolis. Easy ride, avoids various MD I-95 bridge and tunnel tolls. DE 896 is the east-west cutover road from US 13 to US 301, joining US 301 at Middletown, DE. Lots of trucks go that way. Speeds are lower, obviously, but if I've had a tough southbound trek on the NJTP, I enjoy the rolling Eastern Shore countryside. Fresh corn stands in summer!
If I have a specific time to be at KP, I allow 7 hours. It's just, as another poster succintly noted, a crap shoot!