In the eight months that Putin has wasted, Washington has revised its policy governing the use of nuclear weapons.  Washington says its new policy is to use nuclear weapons against threats that are not themselves nuclear threats.  In other words, if Washington believes a decisive Russian victory over Ukraine is a threat to US national security or interest, Washington can use nuclear weapons to prevent a Russian victory.  https://www.zerohedge.com/markets/stunning-strategy-reversal-pentagon-will-no-longer-rule-out-use-nuclear-weapons-against-non

Considering Putin's caution, this would seem to rule out a clear-cut Russian victory.  As the US opposes negotiating a settlement except one on Ukraine's terms, the territory reincorporated into Russia could remain under Ukrainian attack for many years, at least as long as Washington supplies weapons.