How far apart should temporary A braces be on 18ft side walls before permanent bracing?
I would suggest that you brace as required to keep the structure straight and square. The sidewalls will be affected by how straight the lumber is in the first place and also by the process of installing the side wall sheathing. You will need bracing as required to push the shape of the framing into plumb and square and then hold it there.

Also be aware that since the integrity of the structure is not complete until the roof trusses are placed and fastened to the upper plate of the sidewalls you will need bracing in place to deal with forces of nature. As you begin to install the side wall sheathing even moderate amounts of wind can place significant pressure on the wall structures so bracing will be needed to make sure that a strong gust does not come by and flatten all your work.