This message appears for Google Apps users who process large volumes of email.
We've noticed this message has appeared on all three types of accounts (Free, Business, Educational).
A check with Google doesn't provide for any hard, factual data as to what those limits are but from our rough measures is more about size than message volume. That is, the trigger has happened after 200 messages averaging 20M - so 4000M (4G)
We've also seen this notice after multiple days of sustained large data volume, on the order 1.5G/day for three days. Then the trigger kicks off more often.
This issue only occurs over IMAP; a work around is to use the Google Mail web-client.