Use a wooden paddle to keep the membrillo in motion.
Under high heat, keep the material boiling and stirred. As you drag the spoon through the paste you'll note it begins to keep its shape and won't flow back into the void. Keep cooking on high heat.
Eventually, you'll notice as you sweep through the fruit the edges lift off the cooking surface. Keep cooking. You'll notice the surface of the sartén or pot has tiny boiling bubbles where you swept away the paste; the bubbles might be browning. This is the fearless part of cajeta making. Keep cooking and keep stirring. Two hours. Three hours! I usually chicken out too soon. One year I had to re-cook the next day when I was totally dissatisfied with the drying progress.
When you think it's almost done, spoon some into a jar to use as jam. But keep cooking.
Put a dollop on a cool surface and see if you can mold it with your fingers. When the fruit attains a nice plasticity and holds its form, keep cooking another half hour longer until you lose your nerve. If you think it might burn, turn off the heat for a few minutes.
Then cook it some more over high heat, stirring constantly. Test a dollop again. Keep cooking. Turn off, let it cool for a few minutes, then back to the heat. Eventually you'll decide it's done enough, hope you're right. You can cook it until it changes color, or the aroma changes to slightly burned. I bet some people know how to use a thermometer, but I like cooking by eye and nose, it's how I learned. The membrillo I learned from my grandma. I wish I'd paid closer attention when my mom's Uncle Pete showed me pineapple syrup and leche quemada. I remember Uncle Pete telling me to stir the leche quemada until it had the consistency of a baby's spit. He could make camote, bisnaga, pan dulce, all sorts of wonderful food. Now that man could cook! QEPD, Uncle Pete.