Naively without thinking about it. Session Id, setInterval sending a ping every few seconds, clear the id if the ping fails to arrive. Smooth it by taking an average or sum over anywhere between a few minutes to a few days, depending on what you're selling. Some things take more thought and as such those numbers can be quite a bit higher.

There's no need to put any more thought in than that. It's a single design element, and you're not doing a scientific study or collecting marketing data here. This is fine.