If you need more than 15 minutes, just increase the quantity of your order, until you have the amount you need. E.g., USD$120 buys you a full hour, etc.
To check when we can do this chat, have a look at my diary.
You can tentatively set a few options for your preferred date and time after making payment. I will then approve one of these. The booking is complete once you have received an email confirmation from me.