My 1 month old is due for a vaccine. how long after being vaccinated do i need to look out for a bad reaction? an hour a day a week?

Soon: Significant reactions will generally show up within the first hours and less often after 24 hours. Any event more than 48 hours is unlikely to be vaccine related.One exception is a delay of 7 days or so when the MMR can produce fever and or a rash.This is less a "bad" reaction and more a sign the body is experiencing a mild measles from the vaccine virus.