I guess it depends on what you want to do....I've spent 10 days there and found lots that interested me....and I'm not into clubs.
There are museums,3 live theaters, various boating options, historical houses, relaxing by a pool, biking etc.

I'll somewhat repeat what Susie said -- it depends on your personal tastes. Some people could spend four months lazing at the beach every day and partying every night -- me, I'd couldn't imagine anything that would interest me LESS. If you like beaches and parties, four days would be fine.

I stay away from beaches and partying, but my husband and I spend two weeks in the fall and two weeks in the spring in Key West. On the occasional lucky year, we add two weeks in the winter.

During that time, we can opt to do as much - or as little - as we want. As Susie says, there are lots of things to do and tons of wonderful restaurants, too.

Key West is what you make of it!

Key West does NOT have that many beaches to lounge around on. Most of the baking in the sun is done around POOLS.

I agree with the above answers: it all depends on what activities you have in mind and also if you are there over a weekend or not. And you do not have to be into the so-called "club scene" or even the hanging out at on a bar balcony and shouting at the people riding up and down Duval Street to have a good time.

I know people who arrive in November and do not leave until May, but they aren't staying in hotels or looking for things to do.

Most Key West lodgings require a two or three night minimum stay... if that's the case, book two nights in South Beach...

Consider how the nights are booked by what you want to do there... weekends are busier in both areas so decide which area you want to party and when you want to crash and plan the nights...

I prefer to party in Key West and crash on South Beach...

I will probably get slammed for saying this, but I think Key West is a tourist trap. In my book, anyplace with a Ripley's Believe it or Not AND a Hard Rock Cafe is a tourist trap. Not fond of the cruise ships pulling in and unloading thousands of people.

I would stay one night or stay overnight in Key LArgo and drive down and back for the day.Go to the sunset festival at the dock and Hemingway's house.

On the other hand, the ride to Key west is gorgeous as you drive over the tiny islands that make up the keys, surrounded by water.

A few nights in Miami - would be a great add-on: go to Viscaya mansion, SOuth Beach, Little Havanna.

In FOrt Lauderdale, enjoy the beach and Las Olas Blvd at night.