The general recommendation is that a frozen bird shouldn't be left outside the freezer for longer than two hours. I use a thermometer to ensure a defrosted chicken is safe to cook if its temperature is still under 45°F, no worries. The timeframe for safe defrosting can vary a little. For example, if the bird is inside a sealed container, it could still be okay for cooking when left for more than a couple of hours. But at the risk of repeating myself too often - don't take any chances. If you cook a whole chicken, make sure its thermal center has reached a temperature of 165°F (74°C). A Thermapen Mk4 is an excellent device to use for this purpose.