How is it possible to quantify the strength of a Lewis acid/base?
$$\ce{A~  + :\!\!B -&gt; \ \ A\!\leftarrow:\!\!B}$$

$$K_f = \dfrac{\ce{[A\!\leftarrow:\!\!B]}}{\ce{[A][B]}}$$

See sections 4.1 and 4.2 in the following reference for more information:

http://goicoechea.chem.ox.ac.uk/teaching/abse/abse_lecture2.pdf

But $K_f$ is specific to an acid-base pair.  

A more universal scale is formed by the Gutmann–Beckett method by observing the phosphorous NMR spectrum of triethylphosphine oxide as it interacts with a sample lewis acid. 

An Acceptor Number (AN) scale is formed with AN=0 corresponding to triethylphosphine oxide in hexane (P chemical shift of 41), and AN = 2.21 (d-41), where d is the chemical shift of triethylphosphine oxide interacting with the sample. The greater AN is, the more acidic the compound.