What are the earliest and latest dates to apply for a 30-day extension on a single entry visa in Thailand?

7 days is the earliest and obviously the last day of validity the latest @ Chon Buri Immigration last time I checked (May)

How about narrowing down the question by being gracious enough to say what country your passport comes from and what immigration office you will be using for the extension?

But only if you go to Phuket, Bankgok, Chang Mai or Buriram, all others Immigration Offices let you apply for an Extension of Stay between 30 days and your last date stamped as Admitted until../../.. in your passport. Next to this, depending on how busy it is the Immigration Offices in Pattaya and Koh Samui will sometimes even reduce this term to between 7 days before and your Admitted to Stay date. Yep, this is the long version which you do not read any way, so be prepared for when you are told to come back another day.

Between 15 and 60 days after landing.