I have several fields and some of them are busy and other ones are empty so horses can always have nice grass and soil. It's important to let the ground some months to grow back. I remove the droppings and some farmers come to pick them up for their growing. I have wood for the fences and make a treatment when needed. The shelters are also in wood and I also treat them against bugs. I put straw inside and change it once a week. And we check the water everyday.
I try to remove the filth every week (droppings, branches, little stones...). I check water twice a day to be sure it works! And I'm lucky to have several paddocks so when it rains a lot and the ground is soggy, I can put my horses in another one :)
I clean them everyday because if not, work piles up! I remove the droppings everyday and the weeds about once a week or every 15 days. I check the drinking trough twice a day.