Typically, the wall thickness will be in the range 0.5 mm to 4 mm. In specific cases, wall thicknesses that are either smaller or bigger also occur. A basic design guideline is to keep wall thicknesses as thin and as uniform as possible. Where varying wall thicknesses are unavoidable for reasons of design, there should be a gradual transition as indicated in Figure 1.
In general, it’s relatively easy to remove metal from an existing metal mold cavity. Adding metal, on the other hand, can be difficult (expensive) or even impossible (requiring remaking the mold - expensive). Looking at this from the part’s wall thickness perspective, you can make it larger, but you can't make it smaller. So, in case of doubt, start thinner rather than thicker; this principle is called designing "steel safe" or "metal safe."