I got 75 units Botox. I see immediate results (almost overnight). Will the Botox last as long?

Botox longevity varies from person to person, based on injection technique, and based upon dosage.  Often first time patients experience a quicker onsets from botox.  Time can vary from injection to injection.  The longevity of the results has nothing to do with onset.  The best time to touch-up botox is when it begins to fade.  It is better to stop lines before they start creasing again rather than waiting for a specific timeline.

I personally note the immobilizing effect in my forehead the day following a Botox treatment, and it may show most potency over the first 72 hours. The complete effect is typically noted around a week following your treatment. Enjoy.Dr Joseph

The time to start seeing results varies by the individual.  For most people the initial onset after a first time ever treatment is 3-4 days.  Dysport tends to kick in a bit sooner, so for those who need a faster response, we will use Dysport.  75 units is a lot for an entire forehead!  If the amount you were given was truly 75 units, you will probably be good to go for up to 4 months or longer, particularly for the forehead and muscles between the brows.  Crows feet recover a bit sooner.