Bit late to the game here, but.
I watched from about two-thirds of the way through Season 3 of Buffy, until the episode before “The Body” in Season 5 without sleeping (note, I had been up the whole day before realising it was on). I then woke up the next morning and watched from early Season 6 through to “Chosen” nonstop. It was back in April when one of the television channels here in Australia held a Buffy marathon from Season 1 through Season 7 – I was late to the airing.
Back in June the same channel was airing Season 1 of Angel, I watched it all, I think it started at 8.30AM and went through to about 3AM.
I then bought the complete Angel series the following month and watched it from a Thursday through to a Monday or Tuesday. Sadly, I was that annoyed I had no more Angel to watch I then watched Season 5 all of again. I got the complete Buffy the Vampire series back in September for my birthday and watched all seven seasons in about eight days, it was insane, days started to blend together or something of that nature. I slept for a few hours every now and then on both occasions, but that’s all that separated each viewing, it was pretty gnarly, more so with Buffy because it’s such a longer season.