Write down what you prepped. After the session, tick what the party actually touched. It works out which kind of prep you waste, because "I over-prepare" is not useful advice, and "you waste 80% of your NPC prep and almost none of your encounter prep" is.
Add each thing you prepared as its own line. One NPC, one scene, one map : not "the whole tavern arc".
Ask any experienced DM and they will tell you they over-prepare. Ask them what they over-prepare and the answer gets vague, because nobody measures it. Prep disappears into a session and you never find out which pages were wasted, only that the evening ran fine and you are tired.
The useful question is not how many hours you spent. It is which category of prep survives contact with the table. Those numbers are different for every DM, and they are stable enough to plan around once you have three or four sessions of them.
Run it for three sessions before drawing conclusions, one night is noise. Then cut your worst category in half and see if anyone notices. Usually nobody does, and you get an hour of your week back.
A category under 40% is not a failing, incidentally. Some prep is insurance: you write the guard captain because the party might go to the barracks, and the value is being ready, not being used. The distinction worth watching is between prep you were glad to have ready and prep you forgot you had written.
Scrivorn records your session and writes the recap, keeps your campaign wiki up to date, and gives your players a portal to read between games. It also tracks prep against what actually happened, automatically.
See how it works