Straight from Constellation’s website:

† 90-Day Guarantee Period. You may terminate this Contract without incurring an early termination fee during the 90 days following your enrollment date (the Guarantee Period) by notifying us in writing or by calling our customer care center at 1-855-465-1244. Upon your termination during the Guarantee Period we will return you to being supplied by the Utility at your next available meter read date, or if you are a customer in the Atlanta Gas Light area your service will continue with Constellation until you complete a switch to your selected marketer, and you will remain responsible for payment for any energy related costs and charges incurred under this Contract through such meter read or termination date. Not applicable in Texas. Not applicable in Connecticut or Illinois where residential customers are never charged an early termination fee.