In a small saucepan, add the cherry juice and bring to a boil. Fun fact - two full ice cube trays of juice is approximately 1 quart.
Add 2 cups of sugar and stir with a whisk until the sugar has dissolved. Reduce to medium heat and simmer for 5 minutes.
Add another 1 cup of sugar, stir well to combine, and simmer over medium heat another 5 minutes.
Add the last 1 cup of sugar, stir well to combine, and boil for simmer over medium heat another 5 minutes.
Remove the syrup from the heat, stir in the lemon juice, and allow to cool to room temperature. You can now transfer to a jar and store in the fridge for up to 1 month.
Canning Cherry Syrup (Keeps up to 1 year)
Fill a canning pot with a rack on the bottom with enough water so the jars will be submerged by 2 inches.
Bring the water to a boil, add 3 pint-size jars or 6 half-pint jars and boil for 10 minutes to sanitize them. You could also sanitize the jars ahead of time in the dishwasher.
Fill a small saucepan with water, add the jar rings and lids, and bring to a simmer to warm and soften the rubber gaskets on the rings. Remove from heat.
Bring the cherry syrup to a simmer, then ladle the hot syrup into clean warm pint or half-pint jars, leaving ½-inch headspace.
Wipe the rims of the jars clean with a damp paper towel. Place the lids and rings on the jars and tighten the rings using your fingertips. If the ring is too tight, the seal might fail, or the jar might crack.
Lower the jars, upright, into the boiling water in the canning pot. Cover the pot and process for 15 minutes (20 minutes if you live above 6,000 feet). Turn off the heat and let the jars rest in the pot for 10 minutes.
Remove the jars and place them on a towel. When the jars are cool, label with the name "Cherry Syrup", date, and store in a cool and dark spot for up to 1 year.