It really depends on how, where and when you use your service. It is impossible to tell upfront, but here are some examples:
M1000 M or larger there are quite a few users doing more than 60TiB each month.
SSD Series have limited traffic, but on that series as well there is users doing north of 40TiB a month.
Dragon series have limited traffic as well, and typical Dragon series user is "more mild mannered", expecting snappy GUI and speed when required, but not heavy 24/7 usage. Despite this, there are number of users doing tens of TiB each month, as of writing this i see some near 40TiB.
It really all depends on how you use the service, and what your needs are. If you do a lot of public torrents, you are going to seed a lot. If you do quiet older private tracker torrents you cannot expect as much total seeded.
For nostalgia reasons, here is the original text from ~2010:
Depends a lot on your usage pattern, but on average a 2009+ Small does around 1.7Tb on older servers, and on newer servers over 2Tb. 2009+ Large on the other hand has way larger variance, but you could be expecting to see well over 6Tb a month, and some achieve even 15Tb a month. 2009+ Medium varies between 2 to 5Tb generally, but some people have reached 7Tb a month with 2009+ Medium.
1GigE servers do slightly more, but due to HDDs not being able to keep up it is generally only slightly more, but peak speeds being way better.
This all depends on your way of using, your torrents, other peers in the swarm etc. For example seeding latest desktop Ubuntu 64bit image can easily reach 10Tb+ a month, while as seeding Star Wreck from Clearbits will be probably in the few gigs to hundred gigs range, as it is so well seeded.
Let's assume you have 2009+ Small, and you have made a home/indie movie which is in size of 600megs, when you release it assume you get 1000 downloaders, during the first day you will probably do over 100Gigs, and then going lower from there. Let's assume it's total publicity for the first month is in the range of 15,000 downloaders using mostly home ADSL with very limited upload, you might be seeing a total upload of 2Tb or ~3400 seeded copies.