Number of Requests per second

This is extra tricky to calculate but some more detail might help me come up with an indicative figure (if I can face the math);

1) 15k concurrent users (all using the application at the same time)?

2) What tcp idle timeout settings are configured? Are connections likely to be timed out? Do users 'dip' in and out as part of a process or is use constant, a large part of the users role?

3) Are file downloads part of the equation? Are they common/frequent?

4) Do users typically keep the application 'open' all do, do they use it constantly?

5) Are users authenticated?

6) Is the site image heavy?

7) Anything else about the application or user use/behaviour that might be relevant (call center related, 24hr or not for instance)?