This really depends on the size of your deck and the type of wood of which your deck is made. If all of the conditions are absolutely perfect, it shouldn't take you more than two or three hours to completely seal your deck. The biggest issue you're going to run into is waiting to apply a second coat - if the weather isn't perfect, you could find yourself having to wait for more than a day just to get around to putting on your second coat. Ideally, though, you're going to go out on a sunny day when the temperature is high enough to avoid this problem.