I'd like to add: Almost all Russians think the same as Putin regarding what is a number of casualties that will be too much. To the question "what is a number of casualities that makes you think the war is a bad thing instead something that hurts but will bring glory?", their answer is: "There is no such number. Our leader will [read: should] only stop once we reach victory!"
But, they do care about a certain other thing: a number of casualties exists that will be too much for people to want to let their children be forced to the front.
They will not stop supporting the war no matter what the casualties, but they will stop going to the front once the casualities have reached their target number. And that will of course end the war.
Also, there's another relevant number: How many percents of your salary is bread allowed to cost before you start opposing the war. This has to do with the death toll, because the more deaths, the more salary must paid to each soldier, and the more the other industries have to increase their salaries in order to remain in competition for the workforce.







I keep writing this here and there, so I'll reiterate it here.
When the Soviet war in Afghanistan ended, some 70 000 absolutely mental soldiers returned straight from the front, and there were another 400 000 or so that had been rotated away from frontline duty and were a bit less of lunatics but doing bad all the same. Those 70 000 + 400 000 were too much for the Soviet Union of 300 million people.
One of the most important causes for Soviet Union's collapse were those Afghan veterans who were accustomed to extremely violent way of doing things. The crazy years of 1990's and the famously violent Russian mafia were a result of those 500-ish thousand madmen having been freed to roam the Soviet Union and later the Russia. All that instability eventually led to the total economical collapse of 1998.
So... Now there are some 700 000 soldiers more or less on the front, and another 700 000 doing other military duties. Those 700 000 + 700 000 will have quite an effect on the Russia of 140 million people. Once the war ends, ten times as many lunatics will return to the Russia of only 140 million as returned to USSR of 300 million. That will be absolute carnage and the 1990's will look like a walk in a park compared to what's coming up.
This is already unavoidable, but if the Russia is victorious, it can still avoid being ripped completely apart by that carnage. That's the main reason the Russia cannot end the war. It will wage the war ad infinitum, unless made physically unable to continue. And if they some day cannot get any more soldiers, then that'll finally the physical barrier they've been looking for.