'Single Entry Tourist Visa will be valid for 3 months from the date of issue. You must enter Thailand within the validity of visa. However, the permitted length of stay is up to 60 days from the date of arrival in Thailand.'
And, though it should have worked, it went wrong at BKK on my way out as Chiang Mai immigration stamped the wrong frigging date. Always check your stamps right away.
I know people are renewing. And if they can do it once then they should be able to do it twice. That will give you 90 days. I did that once and then applied for a 7 day extension at the end of that period. It still cost 1900 baht though!