Renewal of my Non-O extension is due next month.  

 

Q: How many days before/after the due date may I present myself, duly masked, at Immigration?

 

Thanks 

Depends on what immigration office - Bangkok up to 45 days early - some other only up to 30 days early.  You should appear before current extension ends.

After the day your current extension ends you are on an overlay. You can do it early but not late.