When is ice safe? There really is no sure answer. You can't judge the strength of ice just by its appearance, age, thickness, temperature, or whether or not the ice is covered with snow. Strength is based on all these factors - plus the depth of water under the ice, size of the water body, water chemistry and currents, the distribution of the load on the ice, and local climatic conditions.
There is no such thing as 100% safe ice, so please take caution when confronted with an ice hazard.