I too hope to RF - straight-to-booster, so I'm shooting for 6.
OP -- my dd was a little over 3, I think, when I turned her ff. She had reached the height and weight limits to rf in her seat and we couldn't buy another seat then. She just outgrew that seat ff and I got her a harness booster.
Ds is quite a bit bigger than dd was at his age, so I'm guessing he'll outgrow the same seat before 3, so I'm in the process of deciding what seat to buy him to keep him rf until at least 3yo.
I knew it was better to keep them rf as long as possible, but I didn't know about the higher limit seats when we bought dd's seats...or maybe they weren't even around yet? Now that I know better, I'll try to keep ds rf longer. I almost want to splurge for a new seat now and put dd back rf -- she's 3y8m and although she's big for her age she still has a pretty big head.