For real life testing I personnaly can only say that the official demo page can handle easily 20 simultaneous users. But I didn't seen more users on this page for now.
The problem is how much users the server can manage... if timeout is 2 sec, and there is 20 users it is like 600 visits on normal page per 1 minute... :/ i'm afraid the server could have some problems.
I suggest to tune your refresh_delay depending on the number of simultaneous users on your server. If your server is slow, just incresse the refresh_delay parameter.