This is a difficult question to answer. This all depends on the type of work your engineer will be doing on Commusoft, their device, and the amount of time they're in and out of WiFi areas.
Typically 1GB mobile internet data a user, if they complete 5-7 events a day and around 2 photos per job, is recommended. However, to be safe, more data is recommended to ensure you do not exceed the amount provided on your device.
For Android users, Google may automatically back-up your applications, this can cause excess data usage as it's doubling up. You can turn this off within your phone settings. Turn off automatic restore.