Meaning of the word “by” in context

Question

Although we have limited controlled data on surgical interventions like spinal fusion, discectomy, laminectomy, or vertebroplasty, studies often show some immediate benefit for pain, but this typically fades over time when compared to nonsurgical intervention.In other words, by a few months to a few years post-operatively, there’s no difference in pain outcomes for most common spinal conditions.

Does it mean from a few months to a few years?

Answer 1

This is the meaning of by that we have in this paragraph:

Not later than: by 5:30 PM.

(American Heritage)

"Post-operatively" refers to the period of time that starts when the surgery is finished.

Here's what the sentence means:

At the latest, within three months to three years, the effect of the surgery is the same as with non-surgical treatments.