We had DD rear facing in her radian until just about a month ago - so over a year and a half. It was the Sunshine Kids Radian 65, though so it is a little shorter than the new one we got for DS - the Radian 100.
It was harder to have it RF than DS's Marathon just because it was much lower to the seat. We were able to make it work in the passenger seat, but it was hard to have someone tall riding on that side!