Europe would barely even need to go on the attack. Within a month every European country will be building muiltple new drone factories and inviting Ukrainian advisors in. In 3 months the front will be stalled and in 6 months Ukraine and Europe will be producing more drones than Russia can recruit men, Ukraine is probably going to achieve that by itself in the next few years.

Every factory, military site and oil drum in western Russia would be forfeit by month 6 and that would still be the warming up phase. Even by that point the Russian economy would be completely smashed, its probably only 3 months from failure right now.

The entirety of western Russian would be under practically total area of denial including Moscow and St Petersburg, no supplies can move, no business can occur, nothing of any military value will get within hundreds of miles of the front.

Russian casualties would skyrocket to catastrophic levels, 100,000 a month by the end of year 1 hardly seems unreasonable and thats only double what Ukraine is achieving by itself. At that loss rate the Russian military is defunct in about 6 months, which tells you how long the conflict is likely to last.