Ah, the follies of youth! Back in Ye Olden Days, when life was a perpetual dungeon crawl, we tracked time not at all, calling a break every time the spellcasters got tapped out. Divine casters were praying and arcane casters were memorizing a lot more often than once per day! The thieves (we called ourselves thieves in those days and were proud of it) and fighters would heal to the max possible and would stand guard, cook, scout, experiment with magic items, divide up treasure, and otherwise put the time to good use.
Nowadays this varies quite a bit depending on the in-game pressure. After every combat (defined as enemy contained or party in a controlled position), there's a quick party assessment of hit points. Since we sometimes get macho PCs who want to tough out their damage, it's the healer's player's responsibility to specifically ask every PC and ally for a numerical rating (we like spells like Status that enable the healer to know who's hurting whether they'll admit it or not). Then there's a quick tactics discussion on the next move, which is entirely situational. Once you've infiltrated the headquarters of the bandits, you don't stop till the bandits are rendered a nonthreat, so resting consists of passing around water, trail mix, healing, magic items, and buffs. When exploring a complex of unknown total threat level or conducting long-term hostilities against an intractable foe, finding or retreating to a controllable location in order to rest, eat, etc., is undertaken when the tactical situation calls for it.
When there's a halfing in the party, lunch stops are maintained fanatically. We assume halflings to be hypoglycemic, and missing a meal could be disastrous!
Never, ever do we have characters who are so narrowly defined that they are "useless" once their class resources are exhausted. If the wizard runs out of spells and items, he has a distance weapon, and if melee is the only combat possible, he can act as flanker for the rogue or as triage medic or do something ingenious with his familiar. Recently, when the party (2 wizards, 1 cleric, 1 druid, 1 monk) was involved in ship-to-ship combat with a ship full of casters; my monk spent the entire combat saving the lives of stunned sailors dropped into the drink.
The DM for this campaign tends to give us extended periods of downtime followed by periods of intense crisis activity during which the wizards level up faster than they can scribe spells, so we snatch rest when we can by any means necessary. I'm presently trying to convince him that if we cast rope trick, tie the rope around my monk's waist in place of her belt of giant strength, and then cast windwalk on her, the wizards will be able to sit in the extradimensional space all day scribing spells and making magic items as we race to beat the vampires to the magic spring. (Wizard balloon!)
In the game I run, the arcane caster is a bard and the divine caster can only pray for spells once per day, so things have to go very badly indeed for rests to be based on anything apart from the tactical situation and the diurnal cycle.
My players do much the same.
