We charge a Non-Refundable Exit Cleaning Fee $200 at move-in. I have collected the cleaning fee before I give the key.
I don't know how you could properly enforce monthly cleanings.
If you state it in the lease but do not enforce it during the lease, a legal beagle coudl prove your "standard practice" was to allow non-clean homes.
Some posters make a weekly housekeeper part of the rent, citing that appliances, etc look better longer and moveouts/turnover are simpler because they don't have to catch up.
Overall, I don't care how they live (within reason) but it MUST be clean at moveout.
"Clean" is also very subjective - hard to define and quantify.