Trip length duration is currently limited to 14 days.
It's odd that the limits are just in the data it seems, not in actual limit checks. So when you hit a limit, it's indistinguishable from there being no available flights... which seems very bad for UX.
http://www.google.com/flights/#search;f=EWR,JFK,LGA;t=ORD;d=...
gives: Your search – New York to Chicago – did not match any results.
versus
http://www.kayak.com/#flights/NYC-ORD/2011-10-01/2011-11-01
1347 of 1347 roundtrips shown