I'm allergic 2 penicillin/prescribed cephalexin 4 a cellulitis infection in my arm from a iv. how fast would a serious reaction occur if allergic ?

Minutes to hours: A small proportion of penicillin allergic patients can have an allergic reaction to cephalexin. Drug allergic reactions can occur in minutes to hours if already sensitized. It is possible to develop serious allergic reactions after several days of being on the medication and in some cases even shortly after stopping the med. Allergy testing is available for penicillin but not cephalexin.

Quickly: If you are allergic only to penicillin there is about a 10% risk of allergy to Cephalexin a serious reation could involve difficult breathing and falling blood pressure. This could be a medical emergency. A mild reaction might involve a skin rash or hives and though annoying be mild. If you have concerns be sure and discuss it with your physician first.