That's too long. Russia would run out of juice long before that. You have to remember that the economy is one thing, but there's also fighting spirit, which Russians always had when defending, but rarely when attacking. Many people in Russia think it's not their war and as time goes by and life gets harder there's going to be more and more of them, until finally all who wanted to fight will be dead and the rest will rather migrate than die fighting abroad for nothing.
Or the tanking economy could push more people to sign up to fight because there is nothing else to do and it's the fault of the Ukrainians...
There is no logic to things happening in Russia now. People get arrested for holding flowers. State media is bleating incessantly about bird flu biolabs of the big bad West. A sizeable part of the population genuinely believes that they are defending. 140 million people, there could be enough idiots to feed this war for a long time.
Although the looting "armed forces" don't display much fighting spirit, that's true.