For whatever it is worth or worthless.
I have a board that is quite small since it is targeted to a select group of people.
The ubb stats say:
402 Members
25 Forums
1270 Topics
6148 Posts
Max Online: 10
My bandwidth runs about 1 gb per month.
That includes the spiders( usually two)
And also includes daily backups.
As JAISP stated,
I don't know what you would use it for other than trying to figure out if a new host with limits will meet your needs.
Or possibly you are challenging your existing host.
Since you seem to already be using this product. You would be the best person to determine your bandwidth.
You should have statistics reports in your control panel.
It depends on what the users are doing. Including how successful you are with search engines.
If most of your members are uploading and downloading attachments and pictures your numbers will be much higher than someone that does not.