As an Ultimate member, you are eligible for 1-month's free freeze within a 12-month period. Once you add the freeze, your membership will remain active up to your next payment date and then the freeze will begin. The freeze will automatically end at the end of that billing month.
You can remove the freeze before the month is up, but you will be asked to pay a pro-rata portion of your monthly membership when you unfreeze so you can use the gym again straight away.
Heads up, you need to allow at least 4 working days to apply the freeze to hold an upcoming payment. You may find this article helpful as an ultimate monthly member, can I freeze after the free freeze ends?