How much of an antidepressant do you have to take to OD?

It takes a stunningly huge amount of any of the modern antidepressants to reach a lethal dosage. It's far more likely you'll harm yourself than kill yourself this way.

Before you start considering dangerous methods that may leave you in severe
distress, may I strongly suggest you anonymously call a suicide hotline (800) 273-TALK (8255) if only to see if there's another alternative? And if that doesn't work for you, perhaps you can chat with a friend about this first?