The working times you add to your Calendar will always repeat weekly, so you cannot add any fortnightly, monthly or ad-hoc availability.

This is because most clients looking to book an initial session are expecting the slot they've chosen to be available the following week on the same day, at the same time and in the same location. Therefore, you should only add slots to your availability that you have available to book each week at the same time. 

If you have any existing clients who would like to attend fortnightly follow-up sessions, you can add a reserved slot for them as usual. You would just need to make sure to book each of their follow-up sessions on a fortnightly basis when completing the attendance update after their session each week, instead of selecting the option to book their next session for the same time next week. 

For ad-hoc follow-up sessions, you wouldn't need to add a reserved slot for those clients. You can simply book a session for the client whenever a time and date is agreed with them.