My longest streak was probably 9 straight hours, two days in a row, with about 50 total hours that week (not that it matters, I only remembered that because I played GW more than I worked and I was working full-time, 40 hour weeks). I remember the first night I was grinding my monk from level 13 to 20 after discovering how pathetically easy it is to level a Tyrian toon in Elona, and the second day was just really a grind-fest to get everything I needed for 55 farming.
Although, it still pales in comparison to the old days of Diablo 2 during the summer... 12 hours a day, every day, for 2 months.
