Spinach can disappear into a stew long before the tomatoes have finished simmering. If you want distinct green leaves and a fresher taste, wait until the main dish is nearly ready.
Wash as needed, drain, and add a manageable handful at a time. Stir just enough for the leaves to wilt and cook through. Larger or tougher greens need different preparation: remove hard stems when appropriate and give them more time.
Do not judge all greens by spinach. Kale, chard stalks, and tender herbs have different textures. Cook the parts separately or add them in stages when that makes sense.
After the greens go in, check the liquid. They may release moisture or make a thick sauce feel fuller. A small final adjustment is easier than over-seasoning the base in anticipation of a change you have not yet seen.