Agreeing with everyone here, in addition I would say the SOPs should be written so someone with a high school education can read them without any confusion. Also, it varies from project to project. Some projects require a lot of detail while others you can cut down on the detail. Some projects may be very complicated and a step by step detailed version would help. While others may be simple but your over detailed SOPs may just create confusion and have the person following instructions may overthink. So a general rule of thumb, it should be like a lab experiement. Enough detail to replicate the exact experiment to produce the same results.