When do you need to start caring about matched net lengths etc?
100Mz to 200Mz is a good handwavy guide (but not necessarily!)
How much propagation delay is added by PCB traces?
1nS of propagation delay is added to a signal for every 150mm / 6″ of PCB trace.
Varies between PCB’s.
FR4 PCB is typically 4 to 4.5
RF applications, DDR4 memory boards, high speed FPGA boards might choose to use a special exotic (expensive) PCB laminate material with a lower dielectric constant, e.g. those available from rogerscorp.com.