10-12 players active, with 4 visitor slots.

Running Q3A & OSP.

How much bandwidth is going to be used?

The standard answer applies here:
It Depends

Depends on the servers sv_maxrate setting
Depends on the connection the clients use
Depends on how often it's full versus empty
Depends on whether or not you allow downloads

The easiest determination is what is the MAXIMUM it will use based on the sv_maxrate setting. Simple math:
Rate x Clients = Max rate per second
MRPS x N where N = seconds, minutes, days ... whatever you want to extrapolate out to.
This will only tell you what the max potential is. Many clients will connect at less than the max rate and the server may not be full 7x24.

I wish people would stop making these kinds of posts... Usage is determined by several factors. It is almost impossible to give a figure for what bandwidth any givin game will consume in a month.

I wish people would stop saying its impossible and actually give a general idea on questions like these.

I wish I had a pony.

Did you ever wonder why no one answers the question?

Giving an estimate of bandwidth usage is like asking "how much gas would it take to drive from NY to California?"

Every car is going to consume different amounts of gas depending on what kind of car the drag on it weight and how you drive it and at what speeds IE: Highway or city.

Now are you understanding how hard it is to "guess"?

Yes, I understand it as perfectly as I did before you attempted to explain it.

Its one of those "how long is a piece of string" questions. However, chances are very good that the people who reply "there's no way to tell" have a much better idea than the person asking and providing a general figure with a strong disclaimer (or as we see above, a method for determining a maximum figure) is a great deal more useful than no answer at all.

I would also suggest that its much more reasonable to estimate how much bandwidth a game server will consume as opposed to a given website, not having seen past statistics on either.

I just like to figure 6 KB per client it is inaccurate but it should give you some sort of idea

From my experience it's extremely unpredictable. you're probably looking at about 65 KB/sec.

How can you say that without knowing more of the details?
What if the sv_maxrate is 25000 and it's on a LAN? Not likely, but the point should be valid.
If I said I have a 20 player server consuming 50GBs per month, does that help? Not at all. You don't know what the useage patterns are, what the max rate of the server is, what the client connections are, etc.
I agree with X-Gaming on this. I think I need to stop reading these forums after coffee so I no longer try to explain this concept.
Traveller: The best answer is one you derive from your own experience and conditions. It's very tuneable, so experiment.