Everyone at the tables I play at know this guideline. But, absolutely no one I have ever met follows it. In fact, there are times we have only 1 combat session, zero explorations, and 1 social. In a two-hour game at fifth level, there is no way to have three combat encounters. It is literally impossible - especially if they are actually separate encounters. (Not "and reinforcements come.") Heck, I will go so far to say at fifth level, you could barely squeeze in three combat encounters in a four-hour game.
So if timing like this is accurate, How would anyone ever be able to continue a well threaded story with six encounters in a day?