This type of timer would be based on long resting so (assuming cutscenes stop being tied to long-resting) you can always just not long rest.
So only casters would have timed quests since other classes don't need to rest!!!
This is how the classes are balanced in D&D, yes. Long-rest casters are not expected to be able to use all their spells every fight. Depending on their difficulty, you should get through 3-6 fights per long rest. Part of playing a caster is making judgements about what spells you can risk using during a given fight that you then won't have for later fights.
Right! Exactly! And again, the whole point of scrolls DEPENDS on spell management and limited Long Resting. Same with potions and many other items.
Thanks mrfuji3 for summing up what I was trying to say.