Russia has enough foreign reserves [1] to fight for decades [2]. An unfortunate fact but if they're willing to fight until they run out of money its going to be a while.
600 billion in reserves, much of which they probably can't spend due to sanctions. 84 billion military budget. Sanctions have supposedly frozen about half of their reserves [0]. I expect the situation will continue to worsen for Russia though, in terms of revenues, expenditures, sanctions, and money going missing due to corruption.
0. https://www.reuters.com/article/ukraine-crisis-russia-reserv...