Is there a time limit as to when a town can impose a sewer connection fee?:
no,  if your connected to city sewers you have to pay. check with your city councilman. they probably negotiated a billing delay when they were first installed. even if it was a city oversight. its a service you have to pay for. they charged me xtra sewer when i filled my pool and thats 6000 gal worth, even though it didn't go down the drain. if they can charge you they will. especially if all your neighbors are connected, they don't give you a choice.