The 15 minute workday is more a theoretical problem than a practical one.
I have encountered the 15 minute workday in 3.5 and can say with certainty that it is not just "theoretical" but with the following setup, needed to be actively planned against.
Firstly, this was from an Age of Worms campaign with 7 characters of 15th level most of which are optimized. Now no matter how much you try to optimize a fighter-type, at this level, the wizard is the king of optimizing - archmage, quicken, maximize, empower, craft rod feats, strong spell collection (including some material from the Complete Series and PHB II). What happens then is a big disparity between the melee types and the Wizard and Cleric of the group.
This causes a couple of problems:
[Easy Encounters] - The wizard has enough resources to easily deal with small encounters whilst still having the big stuff ready for something more serious. The melee types can only move slowly (unless they use some of their big items), and so between a couple of ranged attacks and a blast or two from the wizard, these encounters are literally a waste of time.
[Medium Encounters] - At least with these the melee types get a go, but not for long. A maximized "something" from the wizard (be it from rod or spell list) and another big spell generally turns a medium difficulty encounter into melee types doing mop up (not very satisfying).
[Hard Encounters] - Now things become a tight rope in terms of planning encounters. The difficulty level has been ratcheted up to the point where the wizard can unleash some of his big stuff but there is an interesting balance here. Either the wizard goes slightly conservative (meaning the melee types have to burn all their good stuff to contribute, generally using up a heap of their resources so they're not too keen to carry on after the encounter), or the wizard unleashes absolute hell and devestation to the point where he uses up his best stuff and is going to want to "rest" afterwards (with the rest of the party usually agreeing because they've been strained to the max with a hard encounter too). And as I say, it's a careful balance here as well because if you push too hard with the encounter, the weaker characters will be killed too easily.
And so, you are left with a mix of encounters that are either unsatisfying, or on the other hand, big enough to truly test the group but leave them depleted enough that they want to rest.
However, I've combated this with trying to push things in terms of time limits ["you have to do A, B and C before tomorrow otherwise D happens... and we don't want D to happen do we?], but after a while, this can feel forced so you are forced back to "boring" encounters or 15 minute workdays with a single encounter being enjoyably played but with the group being happy enough to pull up stumps unless they are forced to keep going.
The factors involved here are a hyper-optimized wizard, a large party with power disparity, a campaign where there are generally few time-critical missions and high level spellcasters who generally don't run out of medium strength spells. I've found the need to plan additional modules so that the 15 minute workday doesn't happen.
I know that the final module or two are going to test the group in terms of resources but by then, I don't see the big encounters as being as challenging as they could be (kind of strange when you consider what they are going to be up against.

).
Anyway, just popping in to say that the 15 minute workday, despite a whole lot of "internet chat"

saying otherwise is real and possible.
Best Regards
Herremann the Wise