There’s no rule book that says artists need to study the human form, though it’s still standard in art school. Although back before art exploded into the conceptual stratosphere last century there more or less was. Personally I’m largely concerned with narratives and making images that portray people (or animals, robots, creatures, and the like) so studying the human form regularly is a good idea. It’s remarkable how something so familiar can at the same time be an utterly humbling experience to capture accurately. While working from photographic reference is standard practice, observing and capturing with my own eyes reminds me, reminds all of us who do it, how much the lens makes things easier by flattening space. Painting from a photo will result in a painting of a photo, which is not the same as painting from life. Next time you walk through an art museum keep an eye out for art created before the proliferation of the camera and after (mid to late1800s). It’s noticeable. While the logistics of working from a live model can invite excuses (it’s too far, it costs money, it’s crowded, etc.) I try to make a habit of it.
If you’re curious about the variations, standard pose times start short around one to two minutes, which lets the artists warm up. These tend to be more dynamic postures, which personally are my favorite. Then they get progressively longer, into five, ten, twenty minutes, and a half hour with the model sitting or lying comfortably.