It depends on the particular therapy, and the severity of injury to the sciatic nerve.
Nerves can regrow in four-six months to a year. Appropriate movement exercises, acupressure and deep tissue massage, or acupuncture, any kind of treatment that involves movement and the stimulation of the nerves will encourage regrowth and actually speed up regrowth.
There was a story about a husband who massaged the feet of his bed ridden wife who was in a coma for 15 years, and after 15 years, the wife suddenly woke up. The only “treatment” during those 15 years was reported to involve only massaging the feet everyday.