How is 5 hrs being measured and over what time period?

it is a window that starts with usage and continues for five hours.

When I was doing an evaluation using a lower paid tier of Claude, I would have a service send a "hello" ping 4 hours before I started my work day, to reduce my first work-hours session window to 1 hour.

The goal was to have this be more of a "thinking" session for planning the next larger block of work, and then being able to use a lower cost model for implementation.

That said, if your 5 hour window quota is 15% of your weekly quota, this means you can be using 30% or more a day of the weekly quota.

when you send your first message, the timer will start.

In all of them I've seen it's a window that starts when you haven't been using it in a while and begin work OR when your previous window expired. The window starts with a set number of tokens and cuts you off when they're used, it is fully reset at the end of the window (it's not a sliding window).

When you haven't used any tokens for an extended amount of time it reverts to the point where whenever you send your first token is when the window begins.