When she outgrows it. At which point it's safest to move her to a convertible seat and keep her rear-facing as long as that seat allows.
As long as she's rear-facing until at least 20 lbs AND 1 year old, it really doesn't matter whether she's in her infant seat or a convertible seat. My son was in his until about a year because he was so small. My daughter was taller, and with two kids it was difficult lugging the carrier around, so we switched her to a convertible when she was able to sit up on her own in a grocery cart or high chair (and when her legs got too fat to fit in the front carrier, lol).
at 3:17 PM on Sep. 24, 2010