Assuming the seller didn’t completely remove the CEL lamp, then it may come up in a matter of minutes, days, or even after a week. The CEL will come back on in minutes if there’s a problem with one of the critical sensors such as the MAF (Mass airflow). In other cases, the vehicle computer will be conducting several start-up tests during the next drive cycles before triggering the CEL light again. For some issues like transmission problems, the light will only come back on when the car shows transmission-related problems such as transmission slips.