How to calculate the speed of AC signal in different mediums?
In general you would have to do a simulation with the physical arrangement to get an accurate answer. 

For specific geometries such as a very conductive trace separated from a perfect ground plane by an infinite expanse of dielectric, the propagation delay is just a factor of the dielectric constant \$\epsilon_r\$ of the medium. 

\$t_{pd} = 85\sqrt{0.475\epsilon_r+0.67}\$  (picoseconds per inch) 

Significant resistance will add something else to the equation because in general it will become frequency dependent. 

If you are trying to do something where this would actually matter such as designing a screen printed antenna, you may have to do more than just this kind of approximation.