One rule of thumb is that a successful counterinsurgency requires something on the order of 20-25 troops per 1000 population (Goode, 2010). Ukraine’s population is 44 million. Simple division and multiplication yields 880,000 to 1,100,000.
Current Russian force levels positioned around Ukraine are variously estimated at 190,000, and commonly tabbed at three-quarters of total Russian conventional ground forces. This suggests to me that successful suppression of a counterinsurgency (which could be supplied from neighboring territories) would strain Russian resources, both narrowly defined military resources, but also financial.
So, statements that Russia is now well prepared (running budget surpluses, big forex reserves, likely higher oil prices) don’t address the question of whether a long term occupation is feasible, on fiscal grounds.
Caveat: Clearly, the 20-25 per 1000 ratio is at best a rough guess, based on a few limited examples, typically from the distant past. With modern technology, who knows. With a smaller area occupied, maybe more manageable. (Population of the Donbas region around 4 million, implies 80,000-100,000 counterinsurgents).