Thickness of ready-to-roll icing
For each of my (now 16) birthdays, I've had a cake decorated by my mum, with ready to roll icing:



I just asked her. She said:


  I normally go for 4 to 5mm. Any less and it is liable to rip. 3 is the thinnest I can get it.


I think that 5mm is a reasonable thickness (I like icing) but if you can get it to 3, go for that. 2 or less is stingy!