There will be a transition period when well and treated water are comingled at your tap.
Arrival of new water at your tap depends on the rate of water usage, intensity of flushing and exchange of water in the storage tanks. The farther your house is from the water treatment plant, the longer it will take for the treated water to reach your house. Residences close to the water plant may receive treated water within a day of when treated water pumping begins. Residences farther from the water plant may receive treated water within a couple of weeks after pumping begins.