Good suggestion.
Just happen to keep track of the opening/closing dates for Dane and Columbia Counties, and for the winter of 2012-13, here are the open dates:
Dane County: Dec. 20-Jan. 7, 19 days (same for Columbia Co.).
Feb. 4-10, 7 days (same for Columbia Co.).
Feb. 22-March 9, 16 days. (Columbia Co. was Feb. 22-March 10).
Total: Dane County 42 days, Columbia County 43 days.
This information for other counties is probably available through their respective Parks (or similar) Departments. Not sure the state keeps those records, but like you suggested, it would be another interesting set of data and would tend to put things into more perspective.