How long does it take to apply for a COVID visa in Bangkok, and can it be approved on the same day?

Covid visa?

I think you mean a 60 day covid extension, which you can apply for after first having the normal 30 day extension. Needs to be applied for before the scheme finishes .

With the 60 day covid extension, you would be given an under construction stamp, and then have to return for the final approval, circa 14-30 days later.

Note the 60 days added are usually from the day of application (unlike your 30 day extension, from your stamp date)

Note 2, where you go in Bangkok, depends on which scheme you entered Thailand on.

Visa exemption 30 days = IT Square Laksi Plasa, or

Tourist visa 60 days = Chaeng Wattana