Maybe someone can assist me. What should be my expected hours from a 4.8kva 48volt LI battery?

5kva Growatt inverter

1 x Samsung double door refrigerator

1 x defy medium refrigerator

1 x dolphin C1600 aquarium filter

home lights- 10 all led bulbs(4-  6V)

small fish pond pump.

Just as a thumbsuck, to do the approximation properly, one will need the exact power draw from your load, but lets assume its no more than 750W/h

80% of battery (depends on battery type either 80% or 90% max DoD)= 3840Wh

3840/750 =5 hours...Just an approximation.

My nighttime load is about 500W/hr with 3 X fridge's and few lights so, the 750W/hr usage is a good estimate for load shedding usage in a "normal" home.

 

You need to check the spec labels on the equipment for the max Wattage of all your electrical equipment
Without that it becomes a guess at best.

With the fridges you could probably use half of the assumed hours as they do switch on and off.
Once you know the combined wattage multiply it by the amount of hours that you expect to run on batteries and that will be your required kWh. 
Lets assume 600Wh for all this. Then your battery would last around 8hours assuming you run it down to 0% SOC which is not a good idea. Ideally you want to shut it down at 20%.  So 7 hours would be the limit.

Get the Watts first and then you could do a fair calculation.