The closest war to today's Ukraine is the Iran-Iraq war. Ukraine's population (41 Mil) is very similar to Iran back then (38.5 Mil). But put it into ratio, the Ukrainians are more similar to Iraq. Russia today is 144 Million against 41 Mil Ukrainians, That's (3.5:1) almost the same ratio as Iran and Iraq (2.8:1) back then
In Iran's case, their starvation of material due to embargo and non-existent MIC forces them to agree to terms, in Iraq's case the exhaustion of funds. Both sides are not going to run out of weapons anytime soon with Russia expanding its weapons output and Ukraine getting a steady stream of Weapons each week.
People have been falsely predicting that Russia's use of human waves as tactics will degrade their manpower to a thin string of numbers in only a mere one year, but, historical examples such as Iran's own use of stupid human wave attacks disprove this, and Iran uses human wave tactics more frequently than Russians today, still, it would take 7-8 years before Iran is exhausted and recruitment levels plunge. Russia could afford even more recruitment drive than Iran in the 1980s..
In Iran's case, 8 years of war failed to convince the Shia commanders of IRGC to professionalize, rather than relying on Zeal alone. In Russia's case there's a trend of making the same mistake again and again, but there are also clear symptoms of change even though slow. The Ukrainians however is closer to the Iraqis, by being able to professionalize at a faster pace than their enemies.
The manpower underdog, in this case Iraq, just like Ukraine manages to stave off multiple Iranian assaults but the consequence is that they could never reach their desired fielded manpower, due to constant losses, troops rotation etc. Iraqi General Raad Al Hamdani and General Hussein Makki confirmed this in an interview right after IF 2003. This led to the formation of an entirely separate unit from the army, the Republican Guards Corps used as some sort of strategic reserves. Maybe in Ukraine's case, this would be the Azov unit.
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Iran (1980s) usually mobilized around 200.000 men at one mobilization, so I expect similar numbers from similarly populated Ukraine (2020s).
The two countries still have room for imo another 10-15 years of combat before they're utterly exhausted. Off course I'm assuming that during that period, Putin stays president and Ukraine getws steady streams of weapons regardless of whos sits in the White House.