How long does it take to change a tourist visa to a work visa?

I am an American living in China on a tourist visa. I have received approval for my work permit and would like to come to Chiang Mai to change my visa to a work visa. Can you tell me how long that process takes? I will be in Chiang Mai from January 30, 2012 through February 6. If I come to the office on the morning of January 30th, can I get the visa by the time I return to China on the evening of February 6th? Will the office be open on those days (M-F)?