Under Putin, Russia is unlikely to resort to nukes so long as Ukraine doesn't storm actual Russian territory (as opposed to annexed land). If Putin were to be replaced, the new guy will likely be a hardliner, and then it's anyone's guess (although still leaning towards unlikely).

America has already privately contacted all levels of the Russian government to spell out the consequences of nukes, and enough people in government (including Putin) still fear America's direct involvement in this war (rhetoric notwithstanding). No nation can ever be allowed to actually use nukes in conquest; the ensuing arms race as every despot rushes for their own immunity to the American military would be catastrophic.