How about a 12 week comp? this way interest won't diminish over the year and people disappear or even forget esp the non-regulars.
I reckon one year is too long, a shorter time like 3 to 6 months would also keep people interested and motivate them to work harder
I understand where your coming from but I reckon to see massive changes its gotta be 12months, if people don't have the staying power they don't deserve to win.