This is a very complex question.
Data usage is made of 2 parts.
The first is your general internet surfing, a lot of sites now detect mobile devices and server lite pages to mobile devices.
The second part is the data used by Apps. This is the hidden data usage that catches a lot of people out.
It all comes down to how you use your phone, there is no easy was to pre determin the amount of data you will use. For me, 500mb is really small. For others it will me a huge amount.
Regards