First thing ....You shoud ...

First thing ....You shoud know rainfall in 15 minutes or 1 hour in the area of your pit,  So you need  a rainguage for your calculation. (Please visit my FB page Jalmitra Vijay Kedia to make m your own Rainguage; made from 2 ltr empty Plastic bottle of Pepsi)

Here are some calculations for your use:
Volume of 6 X6X6 ft  empty pit is 6000 ltrand of a filled pit( with filter media like sand gravels and brickbats is approx. 40% i.e. 2,500 ltrs only .

Time taken to percolate and empty this 2,500 ltr will depend on Porosity of soil ( below ground)  , where RWH Pit is dug.

So Water percolated in the Ground will depend on How many times the pit got filled and emptied during Rains X 2,500 Ltrs. Pit will get filled from  rainwater coming to the pit.  .

Rainwater will come to the pit from a " Rain catcment area"   a)  during rain and b) even after rain has stopped, if there is storm water in catchment. 

  1 cm rainfall in 100 sq mtr area Cathment area of the pit, is 1 cum water 1,000 ltr water. 

Example :

a) if 1 cm Rainfalls in 15 minutes and the cathment area of the pit is only 100 sqmtr : not more than 1,000 ltr water can percolate in the ground (bcoz available water is not more than 1,000 ltr ). 

b) If 1 cm Rain falls in 15 Minutes and the cathment area of the pit is 500 Sq mtr , 5,000 ltr water is available for percolation and may percolate in the ground if that water ( more than 2,500 ltr) is available after rain has stopped, and flowing in the pit . Now the time taken to percolate 5,000 ltr waterwill depend on POrosity of the soil . it may be 1 hour or may be even 1 day