That's a good question. Traditionally the answer was 48 hours, but that's been called into doubt in recent years with newer research showing it can happen much sooner than that. How much sooner hasn't been answered authoritatively, and it definitely depends on age, overall health, and CHADS-Vasc score. So 48 hours might be right for a healthy 40-year old but not for a 70-year old with diabetes and heart disease. It might be just an hour or less for the older, sicker patient.
It's not cumulative. Once your heart resumes a normal rhythm your atria begin pumping again and immediately clear out the stagnating blood. In fact, this is when strokes happen. This is why they won't cardiovert you if you've been in afib without being on an anticoagulant because if clots have formed in the left atrium, restoring normal rhythm will cause them to be pumped out.
You can take a few things other than anticoagulants to reduce the tendency to clot such as nattokinase, but I wouldn't call nattokinase any more "natural" than anticoagulants like Eliquis, and how much is enough and how effective it is simply isn't known. Substituting any known supplement isn't a reliable, demonstrably safe option.
