How many days before the expiration can you apply for a visa extension in Thailand?

You can generally apply for a visa extension in Thailand when you have 30 days or less remaining on your current visa. In larger immigration offices, such as those in Bangkok, Chiang Mai, or Phuket, the application can be submitted when you have up to 45 days left. It's also possible to apply right up until the last day of your current visa if you meet all requirements.