Depth is usually around 10" to 12", it depends mostly on the hardware you're using, for instance pop-up sprinklers come in 2-1/2" to 8" models, and if you attach these directly to the line you've got to be deep enough to accommodate the line, riser fitting, riser, and sprinkler. If you're using pop-up impact sprinklers, then you'd go even deeper unless you use silly pipe connections, which would then allow a shallower pipe, yet still go deep enough to accommodate the sprinkler.

Silly pipe will work with any sprinkler head, but it does considerably increase installation costs because of the extra fittings and pipe. The benefit is that you can precisely locate and adjust the height of individual heads with ease, so at least with larger impact style heads it's worth the investment.