Russia cannot win this war without delivering a decisive blow to the Ukrainian forces. As of now, Ukrainian Military is holding really well. After the 1st 5 days of war, Russian forces are not making any significant gains and even taking small villages require weeks of fighting. They don't even have air superiority and they are incapable of attaining air superiority at this point. Few months from now, hundreds of thousands of new Ukrainian recruits will join the fight, armed with western weaponry & training.
Unless Russia win this eastern offensive and encircle Ukrainian forces in the Donbass, they are going to loose the war.
I doubt the Russians plan to fight a long war, they are using limited number of troops, after all, and they plan to keep it that way. They haven't used their air force either, so there was not a lot of air superiority missions happening anyway.
There's also the question of whether the Ukrainians will escalate. Reinforcing existing units in Donbas should be fine, but if new units step into Donbas, we do not really know to what extent the Russians will be pushed to respond. There was originally talk of the Russians levelling cities, but that's probably the last option.
I guess the answers will come soon.