For a sense of scale (only scale, money is definitely not the most important criteria), the EU currently spends twice as much on their military as Russia: https://en.wikipedia.org/wiki/List_of_countries_with_highest...
So if (when) American support disappears, I expect Russia to continue to not go anywhere fast while wasting a lot of lives in the process. I also expect this to surprise Putin, as he thinks Russia is a Great Power and therefore can only be stalling if Ukraine is supported by another Great Power and doesn't recognise that (1) Russia isn't, and (2) the EU kinda is, sort of, when it feels like acting with unity rather than as 27 different nations.
The sole reason Russia invaded Ukraine was that it was flirting too much with NATO.
Putin might be a lunatic, but he is not stupid.
> The sole reason Russia invaded Ukraine was that it was flirting too much with NATO.
Which was only a problem for Putin because Putin's world view is that Great Powers (such as Russia, in his mind) should have a sphere of influence, whereas most everyone else thinks Ukraine is a sovereign nation who has the right to decide for itself which treaties it does or doesn't belong to.
Even then, more like begging than flirting; the invasion made it much more likely. Likewise EU membership.