At Payson for sealing sessions you can walk in any time the temple is open and they will add you to a group for however long you have available.
Erika has it right. They just have a continuous sealing session and you just come and go when you like.
Usually it is on the hour or half hour.
See the other two comments
I think you can call to schedule a session at any time.
Sorry for getting back with you later I just barely saw the question you would have to call the temple to find that out.
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