How many times has a check box been checked?

I am basically looking for a push counter type of formula to give me a count based on the number of times the "Survey Sent" check box has been marked as TRUE. The goal is to see the total number of times that the customer was sent a survey. This check box is automatically updated through a series of workflow rules.

My formula works in that it shows "1" when the box is checked, but once it is unchecked, it shows "0".

IF(Survey_Sent__c = TRUE,1,0)