How many days do you need left on your Non-O Visa to apply for an extension at Chang Wattana?

I believe you are confusing the amount of time you need on your current stamp to APPLY for a Non-O visa in Pattaya with the amount of time you need on a Non-O to apply for an extension.

You can walk up the very last day your Non-O stamp is good for and apply for a yearly extension.

In Bangkok and CHiang Mai you can apply for an extension when you have 45 days or less left on your current stamp, other offices are 30 days or less and some are 2 weeks or less.