Usually a few hours. Some folks stay the whole time, others drop in and out. It's very casual.
I also can't omit the groundwork laid by @DopeyBadger, who started the tradition of the DIS list. He is really the reason this exists! He got a bit busy a few years back, so a few of us stepped in to help and eventually took the responsibility off his plate. My DH is a massive spreadsheet nerd, so he tore apart the timing excel last year and rebuilt it with new formulas. If you're interested in the nuts-and-bolts of that, he'll be at DATW with me.