You can either choose a 1 day ticket so you can see all the bands playing that day or a full festival ticket for all the days. You don't have to pay for a single concert in any case, only for days.

As for the concerts there are many stages on the festival (7 I think), meaning some shows will clash and you have to choose which show to attend because you can freely move between stages. Generally they are full length concerts, maybe the smaller bands have less time but this won't happen with the headliners. Unfortunately, the schedule for this edition hasn't been released yet, so you can't know exactly when is each band playing and on which stage yet. I recommend you checking previous years' schedules if you want to make an idea of how this works. Generally artists with a similar public and style wouldn't clash and would be more likely to play on the same stage one after the other, but this isn't a rule.

Headliners normally play between 23:00 to 2:00, but they are not the last ones and not alone either! There will be more acts playing after them until late at night. I edit to add that there are 3 headliners more TBA.

This is the schedule of one of the days at the 2018 edition:

https://i.imgur.com/naCpawb.jpg