When I operated the B737 we used to say the 45 mins was ideal - you could do it in 30 mins but the extra 15 mins allowed for "cock-ups"!
Mind you when we travelled pax on an Irish airline (who simulator we bought time from) which shall remain nameless, I used to joke that they had specially modified B737s which just had an ON/OFF button. The record I observed after pax had boarded, flight deck arrived and we were pushing back within 10 mins!!