It varies. A typical year, locally, will see ice of 8"-12" thick. 16"-18" is pretty thick for Iowa, but I imagine its not unusual at all for Minnesota & Wisconsin.
At 2.5", you can walk on it but its kind of iffy and you really have to be careful. At 4", its safe for fishermen, as long as they aren't standing around in a group. When the ice gets 8"-10" thick, folks start driving their cars and trucks on it.