It depends on the season and the use of the pool.
In a domestic pool, in summer it is usually necessary to clean the pool and check the chlorine and pH levels at least twice a week, and in winter this could be reduced to once a week.
In a communal pool, it is necessary to clean the pool daily during the use season. In the off-season, it could be reduced to one cleaning per week.
As for preventive maintenance, a visual inspection should be carried out from time to time to detect possible leaks or air ingress, and the sand filter or pump pre-filter should be cleaned at least once a week, depending on use and capacity.