when do we need to apply high speed PCB rules ?
A fairly simple and useful rule of thumb: -

Problems will start to manifest when the track length gets longer than about one tenth of the wavelength of the useful harmonic content of the signal being routed.

This is to avoid reflections affecting digital transmission of data and clocks i.e. to avoid data corruptions.

More or less the same for analogue but you can probably get a bit longer on tracks in some circumstances. All this is governed by this: -

A track that is a quarter wave long feeding an open circuit or high impedance load will look broadly like a short circuit at the sending end at that frequency.

This is the primary concern so try and understand this first. Look up quarter wave impedance transformers for additional reading.