Boylan,
I think the 30 min. thing stems from Buzan's idea of 20 min. being optimal study time. His studies showed that our attention span while we study is high in the first 10 min. (we're charged into it) and then drops off until the last 10 min. of studying (we're excited because we're almost done). The rest of time in between our attention span is incredibly low, as well as our retention.
This applies to normal reading as well as PRing. (Actually, someone used the initials PRSys, and I think it's more accurate and helps differentiate, so I'll start using that instead).
I personally don't follow that rule, and rapid read 'till I'm done, or SR&D until I'm done, so I wouldn't worry about it too much.
-Ramon http://razor.ramon.com