Alergic to cipro (ciprofloxacin) so prescribed amoxicillin 500mg for 10days @ 3x per day for prostatits. How soon should I expect to feel better?

Depends. Amoxicillin does not penetrate into the prostate in adequate concentrations to treat prostatitis effectively. It will treat urinary components of infection, which may make you feel better quickly, along with hot soaks, and when less acute, possible prostate massage. That will keep you on your toes.