Anything from 1000 km up is worthwhile, 1000, 1200, 1400 or 1600km. 200s and 300s are good fun but generally not that memorable. 400s and 600s are generally hard work and the second day of a long ride never flows easily but everything seems fall into place from the third day onwards. It probably helps that the longer rides are infrequent and feel a bit more epic.