How long does a gingivectomy take to complete?

Depends... Is the procedure being performed for improving aesthetics, for reduction of overgrown gum tissue or to allow the dentist to restore your tooth/teeth more precisely? Also, will it be performed conventionally (scalpel) or by use of a dental laser? Laser work can be performed often in less than 30 minutes and has faster healing and is often less uncomfortable healing than conventional treatment.
It varies. The time required for a gingivectomy depends upon the amount of time necessary to be anesthetized, the number of teeth involved, the methods used, and the extent of the surgery, not to mention that each dentist works differently and at his\her own pace. Ask your dentist to explain the entire procedure to you.