There's a great service LoadImpact.com  that allows high-load testing of your website. According to latest research from Akamai, visitors can hardly wait more than 6-7 seconds for site to load.

We have used this service to test how one of our clients' website Timotei Kids would perform under a growing load from 1 to 120 simultaneous visitors. As you can see from the graph below, the load times remained stable at 0.3-0.4 seconds throughout session.