100 years to lose to the Reapers?

During a conversation in ME3, Liara estimates it could take 100 years for the Reapers to finish the war. Perhaps I misunderstood, but that seems like an awfully long time. With millions dying within the first couple of days of the invasion, how does Liara think it would take 100 years for the Reapers to win and harvest everyone? They make it seem like there's only weeks left before defeat is inevitable, so her comment was a little strange.

If the galaxy hadn't discovered the Crucible blueprints, I think the war would have been lost within weeks.