Bring the mixture to a rolling boil. After 5 minutes start periodically checking to see if the jam has reached the setting point. You can test this in two ways (and I normally use both).
1 – Temperature. Using a sugar thermometer check the temperature of the mixture. When it hits 105°c it’s there.
2 – Drip a little of the mixture on a plate you’ve kept in the freezer. Then push the edge of the droplet with the spoon. If it wrinkles, it’s reached setting point.