How Long Does Botox Take to Work if Used for Tension Headaches?

It can take anywhere from a few days up to 2 weeks for the treatment to be fully effective. Typically, in my experience, it takes about 2 weeks. The effects last up to 4 months before another treatment may be needed.

Botox been used with good success in cases of migraine headache. Botox action usually takes 3-5 days to work. The duration varies between 2-3 months. 

Botox is used successfully in many patients with both tension and migraine type headaches.  The Botox should take effect in a few days but can take 7-10 days to have maximum benefit.  Result should last 3-4 months in most patients.  Most patients with migraine headaches will still need some rescue medications but studies have shown about a 50% decrease in headache frequency and severity with Botox use.