It depends when you want them. They should last until winter in the ground without bolting. Carrots are normally biennial, and will bolt during season 2. You can harvest them at any time while growing, and can leave them until a hard freeze. They take frost, but not a hard freeze. anow there is another thing you can trry: Put haybales end to end down both sides of the row, and another row to lid it off. Place bales on the ends to seal everything off. This can be done when the first hard freeze is forecast.
This will keep it from freezing, even up to zone three. You can go out and harvest very crisp, sweet carrots all winter, by lifting a bale. They'll even keep up the quality into spring, if you haven't eaten them all. They're far superior to frozen or cellared carrots. You can also use the hay as the next seasons mulch.