I will echo Ironwolf as well.. the majority of my prep work is continuity and 'what the heck are the rules for that?' research...
Meaning about 30 to 40% of the work usually does not get directly used in the session.
However, I have little fear the party taking a sudden left turn as I have a good idea of how the planned adventure changes based on that.
Background/setting work always gets used.. just not always in that session. The town DonTadow mentions can probably be used later on in the campaign.. or even in a completely different game.
Recently I have had problems with attendance.. so this percentage jumped up to the point of making me wonder why I was bothering. My once a month game session mis-fired 2 months in a row.. leaving the group mid adventure for soo long that the players had a hard time remembering why they were even in the swamps! I still spent about 10 hours a month in prep for the session, re-reading the module, altering the encounters to better challenge the eclectic assortment of characters {Druids, Rangers, and a Monk...}
Anyway...