In my previous job I asked my boss if he would let me drop down to 3 days per week (60% salary). To my slight surprise, he was totally fine with it
I did this after working hard for ~1 year at the company. Once you prove your worth, it's in your employers interests to keep you i.e. it would cost them way more if you left for another job with shorter working hours so don't be afraid to ask
If it was doing it again though, I'd request this during my yearly review, and I'd ask for a 4 day week with no drop in salary
There are already companies offering a 32 hours work week though:
Disclaimer: I'm the founder