How long before the 90-day report should you do it?

​If you report by mail, be sure Immigration receives your TM47 package between 15 and 7 days before the 90-day reporting date. If you walk in, the reporting window is from 15 days before to 7 days after your due date. For online reporting, the window is strictly 15 days to 7 days before the due date. Once you enter the final 6 days leading up to your due date, or on the due date itself, the online portal locks you out and requires you to report in person. Note that exact rules and processing can depend on which local Immigration office you report to, but this timeline reflects standard practice at the Immigration Division 1 office.

If you will spend 90 consecutive days in Thailand you can report that from 14 days before until 7 days after if you report in person