I remember this being in the context of video games, and from some time ago (so things may have changed with Steam sales and the like), but it said that something like 85% of sales occur in the first two weeks. Thus the DRM was there to (hopefully) hold out for as much of that initial time as possible.
Anyway, anecdotal story, take with (many) grains of salt.
Anyway, anecdotal story, take with (many) grains of salt.