The deadlift starts, usually, significantly ahead of the squat, maybe 50 pounds more. There's no reason, nor does the book say so explicitly, that you stop pulling every session after just 2 weeks. Lots of people continue to make progress pulling every session for a long time. They also make progress pulling twice a week for a long time, too.
You do not make modifications after arbitrary periods of time; modifications are made as necessary. There is no, "do X for Y number of workouts, then change to Z, etc." You won't find it in SSBBT3 or PPST3. Incidentally, you should acquire Practical Programming.
There is also the point where increases on the squat go to only twice a week, too.
That being said, don't worry too much about the discrepancy between the squat and the deadlift. There's no set ratio and there are a lot of factors that dictate the spread between the two movements.