At least an hour per shell for overall effects. Actual "good god I'm krunked" can be anywhere from 10 - 30 minutes depending on strain and how potent I've made it.
My overall goal when I drink kava though is to sleep, so I don't usually focus too much on time. I just drink, get krunked, enjoy it, and then go to bed. Drinking to the point of no return is always fun too.