The cooking time for quince jam depends on the texture of the fruit.
More than the clock, the texture of the quince and the consistency of the syrup indicate when the jam is ready; the size of the pot and the heat also play a role.
Firm quinces soften more slowly than ripe ones, and the size of the pot and the intensity of the heat also affect the boiling time.
As a starting point, jam usually reaches the ideal consistency 40 to 50 minutes after it begins to boil. It's best to start over medium heat, then reduce the heat and simmer gently, skimming off any foam that rises to the surface occasionally with a spoon.
The most reliable indicator is the texture of the quince: when gently pressed with a fork or spoon, it should be soft but not fall apart. If it's still firm, simply boil it for a few more minutes. Overcooking can cause the pieces to break down and give the jam a consistency similar to orange marmalade.
The syrup also indicates when it's ready. As it thickens, it becomes glossy, and if a drop placed on a cold plate doesn't spread easily, it's starting to thicken. Keep in mind that it will thicken a bit more as it cools, so it doesn't need to be very thick in the pot.
Therefore, 40 to 50 minutes is a starting point and not a strict rule. For firmer quinces, the time can be extended a few minutes; it's best to observe the texture of the fruit and the syrup together.