Viagra usually lasts for up to 5 hours. But don't be alarmed. That doesn't mean every time you take Cialis, you'll have an erection lasting a full day and a half. Instead, Cialis can help you achieve an erection throughout those 36 hours. Cialis (tadalafil) is an oral medication that treats erectile dysfunction (ED). One dose of Cialis lasts longer than a dose of Viagra (sildenafil).