Rule of Thumb 1 - Wave Length consideration for High Frequency Signals

Rule of Thumb : If the length of the distance between the transmitter becomes comparable to the 1/10th of the wavelength of the signal, transmission line effects have to be considered. 

The transmission line causes delay and any radiation from the transmission line can cause performance issues.

For beginners, if some one wants to have a hard and fast rule, any frequency >50 MHz consider as a high speed signal and start following high speed design guidelines.