Not sure if this information applies to all banks in Thailand. But in Krungthai bank, they might ask something else aside from a residence certificate. If you're a foreigner especially if you have a residence in US, they will/might require an additional documents. Few days ago, the bank employee told us on why they ask us if we have an address in US because it is about "avoiding TAX".

If your work permit extension docs are complete, it will be extended within 3 days. They won't accept your docs if it is incomplete, so you will know that it will be extended once they will ask you to pay for the extension fee.

@Zen ****

Don’t worry, my husband had his Non B in the county. You cannot get Non B stamp at the same day. He went back to the immigration office on the date he was told to go back.