I've had it as low as 8MB free without seeing any performance hits. It probably depends on what apps are running vs. what apps you are currently using.
12Mb is generally reckoned to be the safe lower limit. RIM talk about 5.5Mb for BES users but they also say that when it gets there, things are desperate. I generally run at about 20-25Mb on my Curve and my Pearl and never have trouble. My 8900 which has 256Mb of memory is currently sitting at 150Mb File Free.
