How many days before expiration can I extend my 60-day tourist visa in Chiang Mai?

In Chiang Mai you can apply for an extension when you have 45 days or less left on your entry stamp.

You can apply for the 30 day extension at either the main immigration office by the airport or the satellite office they have at the Central Festival Mall

There is no "how many days do you have to have left" rule, you can apply any time from 45 days before right down until the very day your entry stamp runs out..

No need to wait until your stamp is almost running out because they add the 30 days to your current expiration date.