The depth of studs behind drywall can vary depending on the construction standards and practices used in a particular building. In residential construction in the United States, studs are typically spaced 16 inches or 24 inches apart from center to center. Standard stud dimensions are 1.5 inches thick and 3.5 inches wide. Therefore, the depth of a stud behind drywall is typically about 1.5 inches.
It's important to note that the actual depth of a stud can vary due to factors such as the type of framing used, additional insulation, or other construction variations. It's always a good idea to exercise caution when drilling or nailing into walls to avoid damaging any hidden electrical wiring, plumbing, or other structural components. If you're uncertain about the specific construction of your walls, it may be helpful to consult a professional contractor or use a stud finder tool to locate the exact position of the studs.