The bigger the beet, the longer in the heat. And cooking times also depend on the method you choose, elevation, and your particular stove, oven, or steamer. The only way to know if a beet is pleasingly tender is to pierce it with a fork or butter knife. If you meet no resistance, it's ready. Check every single beet since no one is alike, or just the largest for surety. Ignore advice and recipes that offer hard and fast cooking times as a guarantee of doneness (the timing method is amateur and unreliable).