I get the following from the thai embassy in hull
http://www.thaiconsul-uk.com/tourist-visa-single-tr.php
Tourist Visa with SINGLE entry is valid for 3 months from date of issue and allows a single stay of up to 60 days. If exiting Thailand before the 60th day all remaining days of that entry are lost. It may be possible to extend the 60-day stay by a further 30 days by visiting a local Thai Immigration Office. The fee for the extension is THB1900.