Kinda depends on what you are looking to do and how much time you have. Miami has some nice sightseeing but the only thing worse than the crime is the drivers/traffic. Not a good place to be anywhere near rush hour.
The Keys could be seen in 1 day if all you want to do is ride down to Key West and back. If you are into sightseeing, sampling the local dining, having a brew or two and enjoying the ocean then you could easily spend 4-7 days in the Keys. For more info, check out:
Keys Guide
If you are coming to Florida to do nothing but
ride you will probably be a bit disappointed. It is very flat and winding roads are not the norm. The best scenery is along the shoreline and most of that has been developed with homes, condos and hotels.