Depends. It's based upon how much nerve damage has already occurred and the rate of ganglion cell loss. The earlier the intervention, the more likely the chance to preserve vision. It also matters how likely therapy can slow down the progression of disease in someone's expected lifetime vs. Losing sight from a complication of therapy or just age-related loss.
Pressures. This is dependent upon the pressure and remaining optic nerve tissue. If properly treated, glaucomatous vision loss can often be completely arrested.