How often does your software need to be updated to ensure you have accurate airspace and instrument procedure information when you need it?
Wilduke, I update the operating software in my VFR GPS (Garmin 296) whenever they become available (they're free). I update the databases once per year, which I think is more that enough. The airspace does not change much.
As for the "Instrument procedure information" - remember the portables are not TSO'd and should be used for VFR nav assistance only, and although it is nice to have the 296 on board as an emergency IFR navaid, its just that - emergency backup only.
I have a TSO'd Garmin 430 in the aeroplane which gets its datacard updated monthly.
Dr 