I had the same issue but I got a formula now.
I visit my pics thrice to delete them.
The first time (within 2-3 days of shoot) I delete only the duplicates. Restof the pics go into 3 categories Keeper(around 30%), Not Sure(around 20%), Duds(around 50%).
2nd visit is around 2-4 weeks later Here I only visit the Not sure and Duds. Here I delete the duds which I still feel are duds.
3rd visit is around 3-4 months later. Here I delete what I still don't like. I delete a few from the keeper as well.
Though this plan isn't full-proof, because with better equipment and experience the Keeper rate is going up as well.
And again it takes a bit of time, Some of my folders are waiting for a year for the 3rd visit
That may also be because of my new 2 TB drive