Acceptable on and off cycles for fermentation fridge.

I was wondering what some of your on and off cycles were for your homemade fermentor?

 

I have heard that you only want it to kick on about once an hour. I think my chamber has a bunch of "leak" areas, but it kicks on about 20-25 minutes and then off for 35-40. I place my probe on the carboy with tape and bubblewrap and have a temp differential of 2 degrees

 

How old is your unit?

 

I built it from on of those tiny red bull fridges i got for free. Built a 2x2 frame and used 1 1/2" thick insulation. you can see my build here: http://www.homebrewtalk.com/f51/fermentation-box-using-fridge-condensation-203730/

The fridge is 5-10 years old. The whole unit is less than a year