As a general guide you will need to remove and replace the shavings at least once a week. If you have more hens in a smaller house this will be more often but if you have a few hens in a large shed it would be less.
There should never be a build up of droppings or any smell in the house. If you can smell ammonia then it is already at a dangerous level for your hens.
Every month the house will need totally emptying and disinfecting and checking/treating for mites.