DJI said to wait for 25 minutes regardless of what sounds or lights are flashing or not flashing. So that's what I'll do. They also stated the following for reference.
After upgrading firmware, you can find the TXT file which generated in the SD card root directory. The file name with AB means the record for the drone and GS means controller. If the log says Success , it means upgrading process is successful. When Failed, it means the process is unsuccessful and you need to do it again like before. If Abort, it means your firmware version is the latest and unnecessary to upgrade it again. If there are still problems or errors existing, please find and open the hidden folder named MISC in the root directory of SD card. Send that LOG file to us.