You report IF you are continious in Thailand for 90 days.
This count starts on day of arrival as day 1 or day of report as day 1.
The count of being inside Thailand stops at the moment you leave Thailand.
You report yourself in the window of day 75 until day 97 at the Immigration Office where your address is registered as part of this report is confirming your address.
You can not travel around and us a random office, you have to make sure that the hotel or landlord made the address registration at that local immigration office before you make your report.
Fine for not making a report is 2.000 Thb and could be handed out if you visit an Immigration Office, not by border check points at leaving the country.
First report is in person, following reports might be done on-line in the window of day 75 until day 89 at
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90 day reports are FREE of CHARGE.