Out of curiosity - how does Roll20 determine how many hours I've played?

Off the top of my head, I would guess it is how long you are logged into a campaign. Seems daft that they would count the time that I've spent looking for token art and writing things in journals as 'playing'. Nevermind the time that I just leave the thing logged in and do other things. Last time I looked it told me that I had spent 47 hours 'playing' when I know that I have only spent about 8 hours on actual games. A more accurate thing might be to only count the time where you and at least one other person are logged into the same campaign. Still not perfect, but would be more accurate. We check for activity (in other words, that you aren't idle), but otherwise it does count the "daft" time you refer to... just as World of WarCraft wouldn't just track the amount of time you're raiding but instead is your total time logged in (even if you're just running in circles around Ironforge). You're using the application when you're looking for art and writing in the journal. Which is great. We built the application to be used.

Just to add to what Nolan said, it specifically does *not* count time you spend outside of the actual tabletop interface (e.g. browsing LFG, browsing the forums, browsing the Marketplace, etc.) Only when you're actually in a Campaign. And it's just a fun stat that's a useful gauge for us to tell who our "active" users are, and also how much time people are spending in the actual application.