What is the correct pH value of weak and strong alkalis?
There might be confusion as below:

strong/weak "Alkali"(1) - or rather an alkaline solution, which is related to the $\mathrm{pH}$ value. The conductivity and the strength increases with $\ce{pH}$, but conductivity can be obviously affected by presence of salts.
There is not a strict threshold between strong or weak alkaline $\ce{pH}$. It is often considered relatively and/or it depends on context. As a guide the threshold can be between 10-12, context dependent.

strong/weak "Alkali"(2) - or rather a base ( as an opposite to an acid ), which is related to the base dissociation constant. IF a base has too low dissociation constant, so there is significant fraction of not dissociated base ( like ammonia ), the base is considered a weak base. There is ongoing equilibrium $\ce{ BOH &lt;=&gt; B+ + OH-}$, resp. $\ce{ B + H2O &lt;=&gt; BH+ + OH-}$



E.g. 
$\ce{NaOH}$ solution with $\mathrm{pH} = 10$ is weak "alkali"(1), but strong "alkali"(2).

$\ce{NH3}$ solution with $\mathrm{pH} = 11$ is stronger "alkali"(1) then above, but weak "alkali"(2).

Note: For historical reasons, "alkali" is reserved to hydroxides of alkalic metals, so it should not be used for ammonia. As the general term, a base is preferred, with alkalic/alkaline ( along with basic ) as adjective being accepted.