Short answer: it depends.
Personal anecdote: Way back when I had broadband, I was hosting a little site that averaged about half a gig a day (hundreds of discrete visitors averaging 7 pages per visit). It was static content (some rather long articles and tutorials) plus some images to prettify the site. There really was no way for me to estimate how many people would be interested in my site; I did noticed a substantial jump in traffic once my site was indexed by Google though.
Basically, your site traffic is going to be pretty low until you get indexed by Google or mentioned in some news site.
For managed hosting, there's typically some sort of metering involved that says X amount of bandwidth over 24 hours and Y amount per hour, and other stuff about the last 24 hours, etc. If you go over your limits, they will bill you at a higher rate, usually exorbitantly (which is their way of encouraging you to buy a higher level of service). If traffic steadily increases, you'll probably know when you have to buy a higher level of service. This is rather typical ISP behavior. I'm on the soon-to-be-gone WorldNet i495 dialup plan: $4.95 per month for 150 hrs. of access, but $0.95 per hour beyond that limit.
If you get slashdotted, you'll just have to live with a one-time spike in traffic and a bigger bill that month.
Ultra-cheap services (like the old entry-level virtual host accounts at Best.com) will simply shut down access lest the traffic to your site deny access to others being hosted off the same box. GeoCities.com will deny access if your site hits a certain level. Again, this is to encourage you to pay for a higher level of service.
As to photo.net, you should e-mail the folks who run the site. They post here from time to time. A long time ago, when the site was being run off an MIT machine, they had mrtg graphs, but I don't think the new machine has those statistics available.
