How long do you need to be in afib to cause a clot?

48: Usually upto 48 hours,

No specific time: If you are in afib some blood just sits in the atria of heart and does not get circulated. When blood sits it can clot. It is sort of like driving with your eyes closed. Can you do it without having a wreck? Yes, for a while. But the wreck can come anytime. It will eventually happen. You should open your eyes. If you have frequent or chronic afib, you need to see your personal physician.