How do I set Appointments+ so user can select 30+ minutes?

Hi,

I have set the minimum appointment to the time value of 30 mins.

What if a visitor wants to choose more than 30 mins such as 60 mins?

How do I set it up so they are not limited to only 30 min time slots?

NOTE:
some clients/visitors may need 30 mins, some may need 60mins, some may need more. I need to set it up so various time alotments can be selected.

Thanks!

Cosmo