Can you tell me how much time should i watch my 11 week old for symptoms after exposure to strep?

Few days: Symptoms of Streptococcal infection following exposure should generally develop within 1-4 days. Infants are somewhat protected from infection by maternal antibodies ( antibodies they get from their mother's immune system during the pregnancy) for about 12 weeks.

Watchful waiting: 3 month old babies do not typically have strep throat but if your child has fever,poor feeding or irritable then take him to have a pediatrician evaluate child