5-6 months, provided that we talk about a mid-tier 5/10 boomer normie.
First he would be low-inhib and outgoing, in a belief that his pickup strategies from the 1960s still work.
After receiving more rejection, mockery and humiliation (or even outright threats) in the first few days than during his whole lifetime, he would be kind of shocked, not understanding what he's doing wrong.
After adjusting his strategy (nice guy cuckery, moving from Stacies to Beckies and Beckylites), he could seemingly be getting close to scoring a few times, but he would inevitably end up in the friendzone in all occasions, which would frustrate him even more than getting humiliated by Stacy and his Chad rival before, because he would arrive into our world with an honest belief that even guys who cannot casually fuck all women can eventually get married and live a normal middle class life in a stable, loving relationship with a looksmatch girl.
Getting rejected by even these foids would put him in a major depression he never had the chance to experience in his true lifetime. Later he might turn to more extreme versions of the redpill and self-improvement, but the clearly foreseeable failure of these endeavors would make him lose all hope and take his own life.