Most guys will ride the causeway between Ft Myer's and Sanibel. This would be the Fort Myers side of the causeway. You can ride on either the North or South side of the causeway depending on wind direction. Additionally you can ride by the light house on Sanibel (Eastern most tip) off Periwinkle way. You can park your car in the Lighthouse park facility and feed the meters. One word of caution - please mind the tides and currents by the lighthouse as they are moving faster than you think.
Here is some pretty good info from the locals. You should take a quick read.http://www.fksa.org/showthread.php?t=2099
And if you get down there and aren't sure stop in and see the fellas at Ace Performer for some firstname.lastname@example.org
16842 McGregor Blvd.
Fort Myers, FL 33908
I can't comment on the number of good days you'll get wind. The wind on the gulf side can be pretty spotty and like you said is dependent on fronts. I frequently travel to Ft Myers and Tampa in February and March and can usually get 2-3 days on a 7 day stay.
Definitely bring a 14-16m kite and a bigger board. Good luck.