General rule if your sick leave is the result of a non-work related accident or common illness: first three days of your sick leave are not paid. Day 4 to 20 are paid by your employer, after that, the Seguridad Social or a Mutua (insurance company) will take over. In practice your employer will continue to pay the relevant part of your wages, and gets discounts on social security contributions to compensate for that. Either your contract or your Convenio might state an improvement on this general rule, but never something more unfavourable.
For percentages, see THIS LINK in English.
If your sick leave is the result of a work related accident or illness, then as of day one of your ‘baja’.