I 'turn my mind' to it a lot - I spend a lot of time thinking about it. And my conclusion is that there is no way to force the PCs consistently through 6-8 encounters/day without a level of railroading that I'm not prepared to engage in. In the majority of scenarios, either the natural encounter frequency is far lower (wilderness exploration) or the number of encounters is largely up to the PCs (dungeon exploration).
Do you time limit you quests? Surely: 'the BBEG has to be stopped before [time X] or else [bad thing Y] happens' isnt unusual.
If there is no reason to stop the BBEG, why are they stopping him in the first place?
Off the top of my head:
- Rescue prisoner before they get sacrificed
- Escape the dungeon before being trapped inside
- Slay the BBEG before he does something with the Macguffin
- You dont get paid unless you bring the NPC the macguffin before [event in 3 days]. Dungeon has three levels, each with 7ish encounter in it.
- Rival NPC group also seeks the macguffin
- Reinforcements are coming. You need to clear the dungeon before they arrive and make your job much harder
- PC given quest by mentor - as a test of his skill he must complete the quest by time Y to get reward X
- The dungeon randomly teleports around the realm every 24 hours. Get in, recover the macguffin, and get out before then or...
- The demon grows in power every day. The longer you take the more dangerous he becomes!
- The PCs are cursed and need to remove it by midnight by bathing in the magic waters of the dungeon (the Cindarella gease)
- If the PCs can bathe in the magical water by midnight, they get a boon. The pools magic only works one night of the year - tonight!
Etc etc etc.
Be creative. Im sure you can think of tons more.
For what its worth, dont use a time limit for every adventuring day. 6-8 is the baseline. Many AD's will be shorter, and the occasional few longer.
As a side note, this gives me an idea for a thread - a list of ideas for quest time limits to help DMs enforce the 6-8 AD (similar to the above).
Remember - a single level dungeon with 6-8 encounters is one AD. I like to throw multi level dungeons at my party occasionally. For example in a dungeon or adventure containing 20 odd encounters, I would give my party a 3 or 4 day limit to complete it.
In scenarios like this you can limit short rest abuse by random monsters. The PCs know they get 2-3 long rests to complete the adventure, and are unsure exactly how many levels or encounters the dungeon has. They'll refrain from nova tactics, and will be able to pace themselves accordingly over a longer time frame, using only the single time limit.
A seven level 'mega-dungeon' that teleports around the multiverse every week to a different location, and has never had its final level defeated would be a pretty awesome idea for an adventure hook in fact.