Posted a similar question at an Apple discussion group. This was the answer I got, in case anyone else on this forum needs to know in the future -
"Usage will vary. My iPhone is used on WiFi at home primarily. With 2 of my 3 kids actively using the iPhone (my 11 month old is not a heavy user of the data plan, yet), on car trips and such, haven't exceeded 250MB. Any data used on your own WiFi is not going to be counting towards EDGE usage.
1 GB with typical EDGE speeds (90-130) would take about 16 hours of continously downloading while using all of the available bandwidth. Realize when you are accessing the internet typically you are not downloading continuously, nor does loading web pages typically consume all the available bandwidth."
It was also pointed out that you can only download music via wi-fi, or through home computer, so downloading music isn't a data gobbeling factor at present.