This is basically a volumetric issue. you have a mill with a defined volume, and as you put new feed in, it displaces the ore already in the mill.  First you need the volumetric capacity of the mill, this is the volume of the mill at rest where the ore just reaches the discharge level (often about 1/3 of the internal volume).  From this you need to subtract the volume of the grinding media.  You now have the unit capacity.  Then take the volumetric feed rate of the new ore per time.  Divide the unit capacity by the feed rate and you have the nominal retention time.

Say your unit capacity is 1/2 cubic meter and you are feeding 6 cubic meters per hour, your retention time would be about 5 minutes.

Batch grinding, correct?

https://www.911metallurgist.com/blog/grind-calibration-curve

What type of grinding are you doing - batch in a lab, continuous production?

Sizing of mills is a bit of an art form - on the surface it is a volume issue but it is not just a simple volumetric calculation at the core of a good design, there are breakage functions involved, two-phase flow, and length/dia interactions, power...

Regards

Good day. SmartDog, please do note that what you have written can closely be applicable to a Mill with an overflow discharge. A Mill with a grate discharge is totally a different story. Todd is right. It can be a bit complicated as one would have to consider breakage mechanisms that requires modelling and other complex factors.  

http://www.911metallurgist.com/blog/ball-mill-motor-power-draw-sizing-and-design-formula

https://www.911metallurgist.com/blog/ball-mill-design-calculations-how

and more at https://www.911metallurgist.com/blog/eq

I mean our process mill, it is a continuous production. Its about 3000 mm X 4500 mm. Feed rate is about 37-38 tph. We do not know the exact number of balls but approximately 35-40 tonnes that we put in.

For ball milling, you can assume a porosity of about 0.35 to 0.40 (the volume of space between the balls) and you can normally assume the pulp surface level is equal to the ball charge surface level.

So do a crash-stop and measure the volumetric filling of the charge, then multiply that volume by 0.35 or 0.40.  This is the instantaneous residence volume for pulp.

Next compute the volumetric flow of slurry into your mill, as m³/h or L/s (match whatever volume units you used in the first step).  

Now divide the instantaneous volume by the vol flow (cancelling the vol units) and you are left with time units; this is your residence time.