What is the processing time for a 90-day Non-O visa in Thailand?

Now if you're talking about getting a Non-O visa from a thai consulate before you come here, go to the website of the thai consulate you're applying at and read the visa processing time

IF you're talking about an 'in country' 90 day Non-O visa it varies by immigration office

In Bangkok they require you have a minimum of 15 days left on your 60 day tourist visa/30 day visa exempt entry stamp to apply as the under review period is 2 weeks

In Chiang Mai and a few other places they require you to have 15 WORKING days left on your current stamp (really about 21 calendar days)

So I'd say get here and immediately go check your immigration office and get the hand out listing the requirements at that office.