Time off in lieu of payment for overtime can be done only if you have an agreement in place that provides for time off in lieu of payment see (section 10 (3) of the BCEA).

Time off must equate to what you would have paid if you had paid for overtime worked. Overtime worked on an ordinary working day including Saturdays would mean that you would offer 90 minutes off for every hour worked as you would have paid one and a half times the hourly salary for each our worked.
Your agreement can also include the option that allows you to split the overtime worked with overtime pay and time off. This would mean that you could pay the employee one hours normal wage for each our worked and give the employee 30 minutes off for each hour worked.
Remember that Sunday work equates to double time for overtime worked and you would have to allow double the hours time off in lieu of the overtime worked or you could choose to split this between pay and time off as long as it equates to what the employee would have been paid for the overtime worked.