http://docs.mongodb.org/manual/core/replica-set-oplog/#oplog-size

2 things:
1) We should be very clear about our definition of "stale" as we have not yet outlined that when someone comes to this page.
2) This "For example, if an oplog is 5% of free disk space and fills up in 24 hours of operations, then secondaries can stop copying entries from the oplog for up to 24 hours without becoming stale." is pretty random, arbitrary, and not necessarily true.