Thats probably dependent on the hardware and size and complexity of workflows and DB size. Our JIRA install is one of the slowest webapps i've seen in a while. Can't wait to move on to something else.
It varies widely. We have ~300 users, 30 projects, probably on the order of ~20k tickets. Performance has been up and down, occasionally things slow way down and we have to file a ticket and then they come back with some excuse about re-indexing or relocating the instance. Normally it's pretty acceptable, but it feels like there are a ton of a variables that go into its performance day-to-day and minute-to-minute.
I'm at a medium/large company, but without access to real numbers i'd say off the top of my head its probably >500 users across 100+ projects and 10s of thousands of issues.