what is the earth resistance to be maintained while aircraft is on ground and during ground checks for military aircrafts
For the US Navy, according to this instruction:


  The maximum resistance allowable for the cable (MS27574) itself shall
  not exceed 0.5 ohms.  
  
  The cable’s operational (in use) maximum resistance shall not exceed
  10 ohms.


The difference in values corresponds to the loss of quality in the electrical contact at both ends of the wire (clips). Contacts are allowed to be as resistive as about 5 ohms. For comparison hand to foot impedance is about 500 ohms at 100 Hz, meaning the current will be 50x lower in the body than in the wire in case the aircraft is not at the potential of the ground.

You'll find a similar requirement of maximum 10 ohms in: TO 00-25-172, Ground Servicing of Aircraft and Static Grounding/Bonding from USAF.