Depends alot of the following:
Number of total users.
Number of users in channels, a network with 1000 users all in one channel uses way more bandwidth than a network with 10 users in 100 channels. Simply because the server has to send messages from the channel out to every none deaf user.
Some networks have XDCC channels. They consume alot of bandwidth in general.
Here are a few examples of bandwidth usage:
Leaf with 400 users on a 3000 user network eats 60GB/month
Leaf with 1000 users on a 2000 user network eats 70GB/month
So if you want an exact estimate, then its really hard.
If you have an ADSL connection or anything asyncron, then be aware that IRCd use the most bandwidth for outgoing. So a 2048/512 connection would only be 512kbit/s, which can't hold more than 100 users without starting to lag quite a bit. At least thats my experience.
Some claim that the bandwidth is very depending on the IRCd you choose. This is bull:badword:. As the client protocol MUST be the same and doesn't have compression etc. Only the server to server protocol can be optimized on a specific IRCd. So the different IRCd releases use exactly the same amount of bandwidth on a single server setup.