Two types: (1) a general 3-month waiting period, where no benefits at all (except PMBs in some cases) are paid during the first 3 months; (2) a condition-specific 12-month waiting period, where pre-existing conditions (any condition you've consulted, taken medication for, or been treated for in the 12 months before joining) are excluded for 12 months. The CMS sets the maximums; schemes apply them at enrolment based on your medical history.
SA example: You join a scheme in January with diabetes diagnosed in October. Diabetes treatment is excluded until January next year. Other claims (a broken arm, flu) are subject to the 3-month general wait.