Mixed Berry Jam

This is from a booklet that the IGA supermarket produced. It looks really good, and there's no jars required. But there is one problem - it doesn't say how long to cook it for until it reaches it's desired thickness. I'm going to take a guess on this one, since I haven't made it yet, but i plan to soon.

• Ready In: 35 mins
• Yields: 4 cups

Ingredients

• 6  cups strawberries, hulled and halved
• 4  cups mixed berries (raspberries, blueberries, blackberries)
• 2  cups granulated sugar
• 1  teaspoon lemon peel

Directions

1. In a large saucepan over medium high heat, combine all the ingredients. Bring to a boil and simmer until jam is of desired thickness (I'm assuming about 20-30 minutes).
2. Cool and place in plastic containers and freeze. Defrost in refrigerator before serving. (Can be frozen for up to 2 months or used fresh in the fridge within 10 days).