Was there in 2006 so, obviously, some time ago.
Flew into Miami, got a hire car and drive to to Key West which was an experience in itself
We stayed for 2 weeks on Truman Avenue just off Duval Street. The (apparently) 3 star hotel / motel we stayed in was basic but clean and had a pool.
As others have said two weeks is more than long enough to see everything and then some. Some lovely beaches, great seafood and if you get chance go to Sloppy Joe's - was a regular place for us to go for drinks of an evening. Which reminds me, as we came off Truman onto Duval there was a small bar we used to sit and have a few drinks and people watch. On the third night the barman welcomed us, asked "the usual?" Then said so you guys moved in round here now? To which I nodded (being curious) to then find the drinks about a third cheaper than we had been paying. People seem to visit for a few days either by cruise ship / ferry or fly in, spend a few days and fly out.
That said thoroughly enjoyed it