You can make each meeting slots as long - or short - as you like. Just enter the start and end time of each meeting slot as you create it. Do make sure the start time is AT or AFTER the end time of the previous meeting.
You can also create breaks between sessions - either enter a start time that is sufficiently later than the previous meeting, or create a meeting slot with the same duration as the break, and block it by unchecking. This will add a grey bar in delegates' online Schedules, emphasising the break.
If your event runs over more than one day, remember that the meetings slots will be at the same times on each day. If your session timings are different, you will need to enter all of the meeting slots and then block the ones that will not be available on each day.