The most common thawing methods include letting a roast defrost in the fridge or submerging it in cold water. When defrosted in a fridge, a 3-pound roast will take about 16 hours. Or, submerge it in water for roughly 1 ½ hours. Once thawed, a roast will stay fresh in the fridge for 3-5 days. To defrost in the microwave, use the defrost setting and flip every 5 minutes until fully thawed.