Depends on what your loads are and what your beam depth restrictions are to carry these loads to the columns. Almost any column spacing is theoretically possible.
width and height of the canopy will play large roles, as they will determine the loads on each column (both downward and lateral). But generally a 15' to 20' spacing should be a reasonable starting point. I've found that the lateral forces (i.e., wind) tend to control the design of these structures, so this spacing will keep your column sizes within reason.