How about this...compute the rank the links are in, then store that in a column. In another column, track the click throughs. Then after a specified period, recalculate the ranking and reset the click through column back to zero. That would give you the ability to rank them over whatever interval you wanted.
Another option would be to have a 'current period' click thorugh column and a 'historical' click through column. At the first of every month, rank the sites according to current month click throughs, add the current total to the historical total and reset current to zero.