When do I sew on staff sergeant?

That depends on exactly what you mean. (USAF?)

For Air Force, most likely you'll not put on SSgt before 4 years depending on your promotion test scores. If and when you do make it, you'll be assigned a "line number". Which stacks everyone up according to how long they've been in the current rank before promotion. So if you make it your first time most likely it will be about a year after you test for you to actually put on the new rank.

Also, in the month that your promotion "line number" comes up, you put on the new rank on the first day of the month in which you're being promoted.