We can use orbital motions and estimated sizes to determine the average density of an object. Liquid water has a density of about 1 gram per cubic centimeter. The magic number here is 3. If the density of an object in the solar system is above 3, it is a rocky-iron body. If the density is below three, the object is either rocky-icy or a gas giant, depending on its size.
Larger objects have a longer period of internal activity.
There is no number here, since there are other factors involved. This compression is what caused the protosun to start fusing hydrogen to helium. Composition of the planet Radioactive decay of unstable isotopes causes heat. The more radioactive material a planet has, the more heat will be generated. Silicate rocks often contain radioactive elements.