Late in the 4th quarter. (Minnesota has a June 1 nominating signature deadline.)
Link to May 17 Smart Politics blogpost, "Retirements from Minnesota State House in 2010 Currently Down from '06 and '08".
Excerpt: Although the names of Minnesota Representatives who are retiring from their House seats in 2010 continue to trickle in during this election year, there has not yet been an exodus from the legislature that is out of step with previous election cycles this decade.
In fact, a Smart Politics analysis finds that the number of open seats in the state House is down in 2010 (13) from both 2006 (23) and 2008 (15), and is only slightly higher than 2004 (12).