There are several synonyms to describe "as the day is long." Some of them include "endless," indicating that the experience won't come to an end anytime soon. "Infinite" is another synonym that captures the same feeling of an experience that seems to go on and on, without any end in sight. You might also say that something stretches on "interminably" if it seems to drag out forever. "Eternal" is a synonym that carries a similar sense of timelessness or never-endingness. Whatever synonym you use, the point is that the experience being described feels like it will continue on endlessly, much like a never-ending day.