The average grain size of a fully dense ceramic product is measured to be 3 microns. When the material is held at 1000 and 1200 degrees Celsius for 1 hour, the grain size is increased to 5 and 35 microns. Calculate the time necessary to grow the product's grain size to 10 microns at 1000 degrees Celsius. Also, calculate the activation energy for the grain growth of the product.