Set a large colander over a bowl and line with 4 layers of cheesecloth, leaving about a 2-inch overhang.
In a large (6-qt.) pot over medium-high heat, gradually bring the milk and salt to a gentle boil (this will take 15 to 20 minutes). During the last 10 minutes of cooking, stir the mixture with a rubber spatula, scraping the bottom of the pot to prevent scorching.
Reduce the heat to a simmer, add the lemon juice and stir until curds form, 30 seconds to 1 minute. using a small mesh strainer, scoop the curds into the cheesecloth-lined colander. Let drain 1 minute; then, without squeezing, transfer the curds to the bottom bowl. Stir to mix the curds with the liquid in the bowl. use the ricotta immediately, or cover and refrigerate for up to 3 days.