No. It is not fair to say this, because the party responds to the incentives the DM puts in front of them.If the party is trying to take a long rest is it fair to say they have probably burned up most or at least more than 50% of their resources?
A party of experienced players will try to take a long rest at the point where they think they need it. If you, the DM, make a habit of throwing wandering monsters at them while they rest, they will respond by resting more often, not less, so they have some gas left in the tank to deal with the inevitable nighttime ambush. This is entirely rational behavior on their part.
If you want them to push their resources, a better solution is to plan adventures as a dynamic, dangerous "kill zone" with a handful of "safe spots." Any time you're in the kill zone, you're in danger. Eight hours in the kill zone will result in more encounters than the party can hope to survive, so resting there is impossible. Instead, you have to push on to the next safe spot. Because the kill zone is not static--monsters are always moving around--you can't adopt a "clear a path" strategy, either; the path you clear today will be full of monsters again tomorrow.
Alternatively, you can add an element of time pressure. This is simple to do in campaigns where a villain's actions are driving the plot; more challenging when the PCs are the motivators. Of course, as you note, rations can be a major limiting factor... as long as the party doesn't have access to spells like create food and water or goodberry (I have a whole other rant about these spells, but never mind).
There are ways to enforce "6-8 encounters per day," but I agree with CapnZapp--people shouldn't have to come to ENWorld to get guidance on this, it should be in the DMG.
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