Bring to a boil over medium high heat and then drop to a simmer, at medium low heat.
Simmer, uncovered for at least 1 hour, 30 minutes. Stir regularly to ensure that a burnt crust isn't forming on the bottom of the pot. Adjust heat if necessary.
Check as you get close to the 1hour 30 minute mark. Depending on your stove, and the juiciness of the plums, you will either be happy with the consistency, or you may want to continue cooking it down a bit longer. I like it thick and will often cook it down over 2 hours or more.
Let cool and then jar as desired. Or follow desired canning method.