The general rule of thumb for boiling noodles in your instant pot is to use 1 cup of water for every 4 oz of pasta. My recipe calls for 16 oz of pasta so that would be 4 cups of water. The noodles were done cooking in 5 minutes. It does take a while for the pressure to build up but it was pretty fast!  Depending on what other ingredients you want to add to your mac and cheese you can drain the extra water from the pot and easily add the noodles back in there.