Apparently they're currently spending 6.3% of GDP on defense this year. In comparison, the US spent 9.4% during the height of the Vietnam war, the CIA estimates the USSR spent around 10-15% through the late 70s and 80s, and the Nazis spent 25% in 1939 and 75% in 1944.

I haven't really done a rigorous check on these numbers, and all of them are from western sources so take them with as much salt as you'd like, but it's pretty clear Russia is still far from a true war economy and have significant room to ramp up further if they decide they need to. Also, they have a large trade surplus with both China and India, so they have a consistent source of revenue.