How does risetime affect transmissionline effects?
There is a formula for a 1st order approximation of risetime to bandwidth, BW.

for \$t_r\$ (10 to 90%) =  \$\dfrac{0.35}{ BW_{(-3dB)}}\$

Reflections depend on this risetime and not the repetition pulse rate.

There is a formula for propagation time which depends on the speed of light,c reduced by the relative medium constants,   permittivity  = \$\epsilon _r\$ and distance, x,  for non-magnetic, μr=1, 

\$v_p= \dfrac{c}{\sqrt{μ_rϵ_r}}\$    

For coax. a common value for propagation speed is 2/3 c
= 2e8 m/s= 20 cm/ns  

If the rise time > 20x the prop. time to include reflection time then there is no significant transmission line effect.  This is the same as 1/20 wavelength. Some applications neglect &lt; 1/10λ track lengths for the effect of a single delay. 


  Thus 20x/(20cm/ns) means the Rule of Thumb is you can neglect (in most cases ) the Transmission Line effects using 1ns risetime per cm.  ( microwave filters and grid phase power corrections would be examples of exceptions where less sensitivity to error is desirable.)


Other

The most common mistake in computing prop. delay in FR4  is computing the effective reduced dielectric constant, Er(eff) for thin tracks small (w/h and t/h ratios) .  For a ground plane, it is accurate but as the conductor changes from a plane wave to a thin stripline the inductance/mm increases and the relative ϵ  reduces so using a good calculator such as Saturn avoids these errors or get electrical testing paid for on Striplines designs.