I guess you can dig them any depth you like!
But the recommended depth is between 150-200ml and a good rough guide is to dig to the depth of a work boot. I like to dig the main line a little deeper, lawn PVC a standard 150ml and then the black poly for garden beds around 100ml (knowing that there will be another 100ml of mulch being thrown on top)
Really the question is less about a ‘number’ and more about what is sensible. Sometimes in very rocky areas like Two Rocks you just have to get whatever depth you can. Other areas are soft and digging is easy.
Another factor to consider is the type of turf you are laying. If its wintergreen at 25ml thick then you can have your sprinklers sitting level with the sand surface. If its a thicker lawn like Sir Walter buffalo then its good to have the sprinklers sitting 10ml above the sand level.
For help or advice with these issues or to adjust your reticulation call Andrew on 0400044236 or email brightonreticulation@gmail.com