It's nearly impossible to mobilize a large force quickly, or covertly. There would be plenty of warning, especially if the US is involved because there's an ocean in the way in either direction.

If Western nations decide to attack Russia, I doubt the conflict will stay limited to Russia.

• North Korea will probably support Russia militarily very quickly. They're already supplying weapons, they have a close relationship, and they're reasonably secure against counterattack because China would react very badly if NK were attacked directly.
• Iran will join with Russia, but uncertain whether Iran will actually deploy its military in Europe (probably not), or take the opportunity to pursue their own goals in the middle east while the west is distracted.
• China will probably play neutral for awhile, but continue to trade with Russia and sell them military equipment. China is circumspect, they won't jump into a conflict for ideological reasons, though they'll certainly quote ideological reasons in their propaganda. They will join the conflict when it benefits them and doesn't present extreme risk. Most likely they will pursue their own goals in the south China sea (Taiwan, Malaysia, the Philippines) while the US is busy elsewhere.

An attack from the West on Russia will balloon into a global conflict. It will be bad for everyone, even if it stays limited to conventional warfare.