With DD I think I was going for 12 months and despite extreme supply issues, medication, herbs, failure to thrive and eventual use of a supply line and mixed feeding, I made it to just over 11 months. By the time I got the help I needed I think she was getting more formula than breast milk for the last 6 months though.
DS it was 6 weeks, 12 weeks, 6 months...he's 1 in 2 weeks and we're still going strong. My supply issues have been nowhere near as bad (still used the supply line but only about 20% of his intake). I am hoping to go to self weaning now and would love to make the 2 year mark if possible.