The point at which you have consumed so much alcohol, that you are incoherent, have difficulty remembering simple things i.e where you live, how old you are, how ugly you are, how ugly that girl your about to sleep with is. Being 'shit-faced' is usually an experience you only want to try once, and never again.
"Jesus christ!! did I sleep with that thing last night?? I musta been totaly shit-faced"