To put this achievement in chronological terms, we'd estimate that the average length of these songs is, oh, about four minutes. By our calculations, a full playthrough of the Rock Band catalog would take you two days, 18 hours and 40 minutes. Not accounting for the number of times you'll inevitably fail out on "Painkiller," of course.
Congratulations, Harmonix, on this rocking accomplishment! Now, get back to work. That Paul Simon track pack isn't going to program itself.