I did start FF when we started TTC #2. My main concern was that continuing to pump was sending the wrong signal to my brain and making my cycles irregular (between 28-34 days with ZERO EWCM or other indicators), and that it would somehow confuse my uterus and not let anything implant. I started supplementing expressed milk with formula at 6 months, and by 8.5 months I'd weaned completely. At 8 months we started trying and conceived. 5 days after my BFP I pumped my last bottle. I had some nasty clogs, constant let-downs once the PG hormones started to hit, but eventually my boobies deflated.
BTW, my period came back at 3 months PP though, so I'd had quite a few cycles to wake my body back up from pregnancy.