bring it to a simmer
now slowly add
1/2 lime juice
a slow rolling boil until the jam thickens and passes the saucer test.
So what is the saucer test? Grab a small saucer and pop it in to the freezer while your jam is cooking. If you keep casting your eyes over the cooking jam, you will start to see the consistency changing, it will start to look like a thicker rolling boil. If you think it has started to thicken, get the cold saucer out of the freezer and dollop a small spoonful of jam on to it. Swirl it round a bit, and (hopefully) you will see that it has thickened up. Try running a finger through it. If it’s ready it will wrinkle a little and feel thicker.