Appointments plugin, service reservation custom duration

Hi, I have configured Appointments as our machines booking engine (we are a fab lab and allow members to book 3d printers, laser cutters etc.), but I can't find a solution about how to allow people to make a reservation for several hours. The calendar provided allows to only book one time slot for each reservation.
How can I configure the plugin?
Here's our machine reservations page http://bit.ly/2dpIBz4
thanks in advance