The size of your carrots will determine how long you need to roast them in the oven. Keep the skin on the carrots if you can but do trim the greens off if they came with them. There are nutrients in the skin since it has been in direct contact with the soil and you won't taste the skin once the carrots are roasted.Slimmer carrots will take about 40 minutes to roast while thicker carrots will take up to 50 minutes to roast. Test the carrots by inserting a fork in the thickest part of one of the carrots.