ad_hoc said:
A campaign needs to end sometime. If the PCs can't fail, then there is no point.
Uh ... I strongly disagree with you here, and I'll just leave it at that.
I'm going to have to go with ad_hoc here. If the high level characters are trying to stop a ritual that will destroy the world, they better not take their time about it!
Now I'm not so simulationist that I will set a time and if they don't make it, boom goes the world. But I also will not allow them to dillydally. They will have some idea of how long it will take.
For example, I would tell the party that they believe the ritual could be completed in three days if the bad guys have everything they need. So the time limit is set. Three days.
The party heads off. Now say that they blow through the encounters and get there in two. They will find that the enemies started a little earlier than they thought and they are nearing completion. But I will give them some kind of advantage, maybe they can take a long rest before attacking.
If they made it on time in three days, they can prepare themselves with a short rest and attack, but no long rest.
If things go bad and they have to retreat for an extra long rest and get there after 4 days then the enemies will have started a bit late, but the ritual is just about complete. Time to charge in with everything you've got!
Finally, if they insist on taking a long rest after every fight I will inform those with the proper knowledge skills that they know they will not make it on time if they keep resting. I won't force them to do anything, but after the 4th or 5th long rest. Boom. World changing event. Welcome to the aftermath!
Again, I wouldn't just spring it on them. I would be telling them flat out that they aren't going to make it every time they stop for a long rest. And it isn't even out-of-game knowledge. The characters would
know that they are running out of time.
Honestly it has never been a problem with my groups. They know there is a deadline and weigh that every time they consider stopping for a rest. They don't just assume that the world will wait for them to get there.