From a retired anesthesiologist-
With Covid-19 cases, I have had the same question myself. There really can be no accurate answer for a specific patient in a specific hospital because it depends on how sick they are, how frail, and maybe how early in their course you decide to give them that support. If they have been holding their own with the viral pneumonia but then deteriorate because of a simultaneous bacterial infection that we can treat, supporting them with a ventilator while treating the bacterial infection makes sense and buys the time they need to survive.
With Covid-19 cases, the only specific data Iāve found is that survival rates for patients requiring mechanical ventilation is around 65%, decreasing to about 50% in the elderly. I donāt know the source of that data, though, so itās not really useful for all the reasons listed earlier.