How much can you stand?
The answer to your question depends to some degree where you live. If your'e on the lake in northern Wisconsin or Michigan then your season could be a month shorter than in Chicago. Besides the obvious drop in temperatures that the fall brings there are changes in the prevailing winds and their strength to consider too. In the spring time the really nice, somewhat reliable sailing weather doesn't arrive (IMHO) until late May most years. It can get really nasty trying to sail in the southern reaches of the lake after the wind has been blowing out of the north over 300 odd miles of open water for a few days and the temperature's at 40 degrees. This kind of pattern can be pretty persistent early and late in the season. Something else to consider is how empty the lake gets (boat-wise) after the first or second week in October. You might well be the only vessel on the water for many miles around so help can be much farther away than it is on a warm summer weekend. At the extreme ends of the season the days are shorter, too, limiting weeknight sailing if you're not comfortable being out there in the dark (and many times fog). In a nutshell, six months of great, good, and marginal sailing max. But when it's great it's GREAT. For me, I'm in around the middle of May and out in the middle/end of September. (It was longer when I first bought the boat.)