From experience it is a monitoring process. You can usually get a good feel for the amount of traffic a site does by observation. The problem is, unless their is historical evidence, you can't calculate accurately because you do not know how many people are going to visit the site.
Maybe this will help. Take a look at the site, see how many images are viewable, if they are encouraged to be viewed, etc. Check to see if there are videos or other large files as well. Check the PR of the site with google, and make other traffic observations. Then multiply. But, you have to make alot of assumptions this way. Also, you will need to take into account the number of times one person will return to the site.