How do i know if i have to stay in hospital for an allergic reaction to penicillin?

Severity?: it depends on how bad the reaction is. If you only have a rash and no instability of blood pressure etc you go home with some steroids if it was an anaphylactic reaction until you are stable.

Under a day: A severe anaphylactic reaction may warrant an overnight stay but most reactions should subside in a few hours. The only exception is when there is Stevens Johnson syndrome which may require many days to hospitalization.