High bulk capacitance (100.000 uF) discharge time on Wifi router supply
Is there a way to "have an idea" of the number of seconds the routers
  can remain ON after the AC goes down?


You want to hold the 12 volt DC on for a period of time using a capacitor so, the first limitation of this idea is that the voltage will instantly start to droop once the power is removed. But that isn't necessarily a show-stopper if the router can survive all the way down to 10 volts. It will be taking a roughly constant current as the capacitor is supplying the rapidly drooping voltage and this might be (say) 1 amp (just for numerical convenience). 

The formula: -

$$I = C\dfrac{dv}{dt}$$

So, dv will be the change in voltage (say 2 volts) and dt will be the time allowed for it to droop (say 2 seconds). Hence: -

$$1\text{ amp} = C\dfrac{2\text{ volts}}{2\text{ seconds}}$$

Or, rearranging, C = 1 farad.

That's quite large (supercap sort of size) so pick carefully. If your current is only 100 mA then a 0.1 farad capacitor would hold up a drooping supply of 12 volts for 2 seconds. If only 1 second is required then 50,000 uF would do the job.

But, it all comes down to how low a voltage can you tolerate on your router before it gives up the ghost?


  One idea is to place 10x electrolytic caps of 10.000uF x 16V on the
  12V output out the adapter (supply input of WiFi router), a nominal
  100.000uF capacitance.


Very unlikely to be anywhere near enough. You are probably about a thousand times too small.