More often after the second dose
Usually within a week of vaccination
Most patients with myocarditis or pericarditis who received care responded well to medicine and rest and felt better quickly.
Patients can usually return to their normal daily activities after their symptoms improve.
Those who have been diagnosed with myocarditis should consult with their cardiologist (heart doctor) about return to exercise or sports.
Should I Still Get Myself or My Child Vaccinated?
Yes. CDC continues to recommend that everyone ages 6 months and older get vaccinated for COVID-19. The known risks of COVID-19 illness and its related, possibly severe complications, such as long-term health problems, hospitalization, and even death, far outweigh the potential risks of having a rare adverse reaction to vaccination, including the possible risk of myocarditis or pericarditis.
http://www.cdc.gov/coronavirus/2019-ncov/vaccines/safety/myocarditis.html
No secrets there. The CDC explains it fully.