How early can I apply for a one-year visa extension at Samut Prakan Immigration?

I could not get any information by phone because the number I called for Samut Prakan Immigration office was set in automatic menu loop until it eventually hangs up. so I went there today to get it. My 1 year extension expires at end of April and I have to back to the US at end of March to take care of a few things. I knew that most offered the extension at 30 days before expiration. And few at 45 days before. FYI , Samut Prakan immigration allows extensions 45 days before expiration. Just wanted that info out there for anyone to use it.