I've heard of the Romans losing entire armies and still continuing a war effort, to where the Soviet Union took even fewer than 10% casualties in the afghan war before capitulating.
If era isn't what defines acceptable losses, what historical factors can be used to determine a particular nation's tolerance for military losses?
Incidentally the reason for the USSR withdrawal from Afghanistan which was a military quagmire to be sure had more to do with Gorbachev attempting to reform foreign relations, among other things. Withdrawing from Afghanistan would ease tensions in the cold war and also help normalise relations with China.
As for your question I would say the idea of acceptable casualties depends on the war itself rather than the time. What are the stakes of the conflict for the powers involved, how popular it is with the public etc
Napoleon famously stated "you cannot stop me, I spend 30,000 men a month" which isn't a lot compared to the world wars but I think captures the idea of what you are looking for. France during the revolution and Napoleonic wars were able to sustain far more casualties than any other power in Europe. This was because the revolution installed in the population ideas of nationalism that wasn't as present in the other armies of Europe. There for the level of casualties they could sustain was higher because the population was far more willing to fight for the ideas of the revolution and France.
It depends on the stakes of the conflict and the types of governments involved. Despite efforts by scholars such as Kecskemeti (Strategic Surrender,1959) to find "breaking points" (war deaths as a percentage of population) at least some countries exceed the supposed breaking points where they are authoritarian and fighting for survival (Paraguay in the War of the Triple Alliance is the most famous example). Democracies, on the other hand, are less tolerant of casualties, although still willing to suffer them in perceived self-defense. They tend to fight shorter wars and suffer fewer casualties than their autocratic opponents, after controlling for other factors. Compare US and North Vietnamese/NLF casualties, for example, or the US vs. Japan.
If we're talking U.S, war was seen as "glorious" and necessary until world war 1. Same for the other countries involved in that conflict. Vietnam also has this effect because we weren't fighting a clearly "evil" enemy that wanted to take over the world. We were simply getting rid of communism because we thought it was wrong and because Russia/Cold war. Vietnam was also the first televised war, bringing the death counts to everyone at home via TV. This obviously made it unpopular with the public.
To add to this, the large amount of firepower used in WW1 necessitated the employment of a huge number of divisions (Britain: 5 in 1914, 12 times as much in 1918).. Both required a huge amount of resources which put pressure on the homefront which led to social as well as political changes (strengthening of the left, working women etc). This lends weight to the observation that the perception of war changed significantly. Also, while WW2 was the general turning point regarding disease fatalities, the British troops suffered more combat fatalities than casualties arising from disease in WW1.
Any other country or empire in Rome's position after losing 70k men at Cannae would have surrendered or at least went to the negotiating table to broker a peace. However for Romans(Same with Russia in WW2) it eventually became a much more personal war and they had the willpower to fight to the last man and they wasn't even going to consider surrendering. This belief is usually imprinted in the population through ideas and culture.
Soviet Union could have destroyed Afghanistan in a few days/weeks if they wanted to, the withdrawal was more to do with political reasons. It's the same reason the USA couldn't hold control of Iraq when obviously if they wanted to they could raise the entire country to the ground in a few days. These types of wars are not personal or for survival and objectives are different. Also very hard to differentiate between civilians and enemy fighters and it takes time digging them out.
Sorry I couldn't structure my answer properly, but there is no single main answer to the question, and really "acceptable losses" have never really changed, when a nation is facing annihilation the losses don't really matter as there is nothing to really lose for the defenders, this is seen when Russia went from almost losing Moscow in 1941 to capturing Berlin in 1945. At the end of the day it's all relative to the objectives and circumstance.