Free: show free requests usage counter, as usual
Unlimited: do not render at all
Fair:

• within fair usage: either not render at all (as unlimited) or render brief description/mention to fair usage policy, link and icon?
• over fair usage: render usage counter (showing over 2500? limit) + brief description/mention to fair usage policy, link and icon?

August 26 Update:

Sanja designed the fair usage messaging on the sidebar, on a different location, so maybe we should just hide the usage panel on the sidebar when the plan is unlimited, and let the other places handle the messaging.

Renaming the task from "Sidebar: decide what to do with usage counter" to "Hide the usage panel when the plan is unlimited".