Standard length screws/bolts are supplied with every Gripit. You can use a longer one if necessary for the job, however the screw should not touch any solid material behind the plasterboard. Do not use a screw shorter than the below recommendations.

Yellow - Wood screw (4mm diameter, min. 25mm length).
Red - Wood screw (5mm diameter, min. 30mm length). 
Brown - Bolt (M6 diameter, min. 25mm length).
Blue - Bolt (M8 diameter, min. 25mm length).