How painful is a spinal fusion?

Varies. This varies as to the diagnosis, your age, the extent & location of your surgery, what type of procedure that was done, whether you smoke & if this is your first back surgery. It also depends on how tolerant you are to pain medication in terms of how much & how long you may have been on medication preop. Depression can also magnify pain & if in pain for 6 months or longer, higher risk depression.
Not uncontrolable. Dosent hurt as bad as bad as knee replacement hard to get out of bed but sitting uncomfortable for two months but controlled on pain meds every body is different.