how to calculate the average key search for AES-128bit?
Since key search is "embarrasingly parallel", i.e., you can have different ASICs searching different parts of the key space, you have $$20{,}000\approx 2\times 10^{11}$$ processors and the keyspace is of size $2^{128}$.
There are $$3600\times 24\times 365.25\approx 2^{24.9}$$ seconds in a year so you can check $$2^{11} \times 2^{24.9} \times 2^{28.9}\approx 2^{64.8}$$ keys per year, the last factor being $2^{28.9}\approx5\times 10^8$. So this gives
$$ 2^{127} \times 2^{-64.8} =2^{62.2}\approx 10^{18.72} \approx 5.25 \times 10^{18}$$years if I haven't made a mistake somewhere.

Also I interpreted your 


  can search 5.108 keys per second


as $5\times 10^8.$

Edit: Fixed computation. Thanks, @dave_thompson_085