Sperm banks and fertility clinics usually tell sperm donors to go 36 hours without ejaculating before making a donation in order to give their sperm count time to build back up. For most men, it takes between 24 and 36 hours to “replenish” all the way after ejaculating and get back to their normal sperm count. However, the testicles are producing new sperm all the time. Even five minutes after a man’s last ejaculation, there are a few sperm ready to go. Views · View Upvotes.