Wetsuit diving in WI

[maremd50]

This is my first spring since I've started diving.  So far, all my dives have been in tropical locales - all 20 of them!  I have a 7 mm wetsuit, and have already ordered an additional 3 mm shorty for layering and extra warmth.  I've also located a Hotsuit Wetsuit Heater, (which is a belt-type thing that gives approximately one hour of heat by breaking an enclosed tablet, which causes a chemical reaction) but am wondering how much local diving I can reasonably expect without going into a drysuit. 

My question is this - How many of you dive in a wetsuit - locally?  When can I expect that some of the inland lakes will warm enough to tolerate diving in a wetsuit?  Also, to what temps do the inland lakes get?  I appreciate you sharing your experience with a newbie.