There is no rule about adding hops for an hour. While 15 minutes does seem a bit short, using hops under 20 minutes is called the 'late-hopping technique' and is used by many prominent brewers. It provides a less harsh bitterness. A fifteen minute addition will give you roughly half of the utilization of a 60 minute addition, so you need to increase the amount of hops you use.
Brewing Better Beer, 66-67
Also, I'm sure the can of extract is already hopped. Good luck on your brew! I think 2 oz dry hopping will be fine. It will be prominent, but not overpowering.