How much pain should i expect to be in after getting spinal fusion surgery?

Depends: It depends on the type of fusion (how many bones are being fused, what part of the body, how the surgeons "gets there" [front, back, side, combination, "minimally invasive"], your pain tolerance). Ask your surgeon. Consider a second opinion if you are uncertain. Many fusions are elective (not an emergency).

Varies: Varies on the type of fusion & approach, number of levels, whether you are a smoker, your overall fitness & condition as well as health & you age. The size of your surgical incision plays a role especially if minimally invasive or not & the underlying reason for your surgery in the first place as to it's success rate in relieving pain if that was one of the reasons for the surgery to begin with.