During the week, you should ideally perform your hefsek taharah a few minutes before sunset and immediately insert your moch dachuk until after nightfall. In your example, you should try to begin your hefsek taharah by 6:12 and keep your moch dachuk inserted until 6:57.
In some communities, the custom is to leave the moch dachuk in for shorter periods after sunset. We suggest that you check with your local rabbi about this. Concerns about irritation from the moch dachuk can also provide grounds for leaving the& moch in for shorter amounts of time.
An alternative is to perform a hefsek taharah earlier in the day and then perform a bedikah just prior to sunset, without removing the cloth – the bedikah cloth remains inside and serves as a moch dachuk.
When planning a& hefsek taharah close to sunset, it can be helpful to perform a bedikah earlier in the day as a back up. (Please see our article on timing of hefsek taharah for more information.)
Please note that insertion of the moch is an ideal, but its omission does not usually affect the validity of the hefsek taharah (see our article on the moch dachuk for more information). Additionally, if a woman has chronic problems with the moch dachuk, she may be given permission to omit it.
For a discussion of timing on Friday night, please see our posted question on that topic.