Heat 8 cups of whole milk in a large saucepan over medium heat, stirring occasionally. Watch for small bubbles to form around the edges—do not let the milk come to a full boil.
Remove the pan from the heat. Stir in 4 tablespoons of freshly squeezed lemon juice (or white vinegar) and a pinch of salt, if using. Gently stir just once or twice.
Let the mixture sit undisturbed for 10 to 15 minutes so the curds can form and separate from the whey.
Line a large colander with cheesecloth or a clean cotton kitchen towel and set it over a bowl. Carefully pour the curds and whey into the lined colander, allowing the whey to drain into the bowl.
Let the curds drain for at least 30 minutes for a soft ricotta. For a firmer texture, let them drain for 1 to 2 hours, or gently squeeze the cloth to release more whey.
Transfer the ricotta to an airtight container and refrigerate for up to 4 days. Save the whey for use in bread, smoothies, or watering plants.