How far do the cabinet doors go into the internal space when they are folded?

This would depend on the width of your cabinet door, the max width allowable with this system is 600mm, in this instance the doors would go 600mm into the cabinet, inclusive of the track width. With regard the width these doors would take up within your cabinet would depend on the thickness of your doors and this total size taken up would be the thickness of one of your doors multiplied by 3.