It takes on average between 6-12 hours for car seats to fully dry out. The exact time depends on the weather and the initial condition of the upholstery. In summer, it's enough to have your windows open while it's drying, and the sunlight and heat will do the heavy lifting. In winter, it is very important to cycle between windows (and possibly doors) wide open, then windows slightly open with heating switched on, over and over until it's fully dry.