The jam is finished cooking when it has thickened and reached 215-220 degrees Fahrenheit on your instant read thermometer. A simple test you can use if you don't want to use a thermometer is the cold plate test. Chill a plate in the freezer before you start making jam. When the jam seems to have reached the proper consistency, place a dollop of jam on the plate and let it stand for a minute of two. It should start to gel and shouldn't be too runny. If you run your finger through the center, it shouldn't run back together.