As I understand it, the Russians, especially on the northern route, would have to move in February before the ground thaws and turns to mud.
Putin may have gotten himself into a Catch-22 pickle, though. This isn't Georgia, where the West didn't care and wasn't watching, anyway. The Ukrainians, while no doubt understaffed and under trained, have been supplied with some nasty weaponry from NATO countries, including Turkish drones, Stinger anti-aircraft missiles, and multiple anti-tank weapons.
The Russians would sustain major casualties, and the Russian people, already sick of Putin's endless mafia-ocracy rule just may not have the patriotic rah rah about their conscripted sons dying for Putin, just to make sure that his autocratic clique is protected from a potential uninvade-able NATO protected democracy as an example for their own freedom.