Wolf... the most common time frame is the past 12 months, but that's just what's "common"... not always what's done. Tell ya what: try to work with your current lender and see if they consider removing some or all of those lates. Countrywide Home Loans has done it for many customers (me and others) and I would imagine other lenders would consider it, too.
If your lender is interested in keeping you and refinancing you, chances are they'll be fairly flexible. Go for it and see what happens!