Hey how are you?
I have the following scenario:

One organization

    Exchange 2010 Sp3
    Active Directory 2008 R2

Two organization

   Exchange 2013 SP1
   Active Directory 2012
   FIM 2010 R2 SP1

through FIM 2010 synchronizing contacts?
My question is how much of acho band used to synchronize each user?

The answer depends on several things

1) How much data does each user take up in AD (which depends on which attributes are populated and how big the data is in each)? Suppose 4k

2) Which of those attributes are you synching? How often will those attributes change? Suppose 1k worth of changes per user per week

3) How often do you plan to run Imports and exports from and to these directories? Suppose twice a day

Vague answer: not a lot.

Specific answer for this scenario would be about 1KB per user per week plus some overhead during those times you export and import.

Your answer will of course depend.