Not very much - most of our clients use less then 250MB per month per iPhone/iPad.
It varies from client to client. Because ServiceM8 is used across a wide range of industries and businesses, everyone uses it differently, and makes it difficult to provide a simple answer. The amount of bandwidth that you use will vary heavily on how many photos you attach to jobs, how many documents you open from jobs, and how many active jobs you are running at any given time.
Your iPhone can actually tell you how much data your using. Simply open the Settings Application. Click General. Click Usage. The key figures you need to watch are your Cellular Network Data.
ServiceM8 will further reduce how much bandwidth your phone uses through some smart technologies. For Example, if you are near a WiFi access point (such as your office), ServiceM8 will automatically update all of the active jobs, download all current photos and any other information available so that when you use it in the field, your photos are ready to go - saving you bandwidth and time. The other bonus of this is that in regional areas where you might not have network access, you've still got your client and job details handy.