Assuming a fib is common, at what point should you seek medical attention?

Common but important. Atrial fibrillation is common affecting 10% of people by age 70 and increasing in frequency with age. If certain risk factors are present, it can be an important and preventable cause of stroke. Certain conditions such as high blood pressure, thyroid excess, and mitral valve disorders cause af and need treatment. Thus, everyone with af should undergo a full evaluation at least once.
Immediately. Atrial fibrillation is a serious diagnosis that requires thorough evaluation and close follow up with a physician. Blood thinners are recommended for the majority of patients with this condition and the dose needs to be monitored frequently.
Now. If you know you have a atrial fibrillation it sounds like you've already sought help. If not, please do.