how can I calculate the amount of capacitive power (kvar) to improve power factor of the given load? 1125 kw load for almost 3 hours

Gagan,

You can't without more information such as the Power Factor (pf) or load KVArs. If as you state the load is 1125 kW + j0 kVArs then by definition your pf is 1 and you do not need any compensation, Time is irrelevant unless the utility tariffs penalize you for poor pf on a demand/time basis.

Alan

If you are correcting the PF of an individual motor, it's best to obtain a recommendation from the motor manufacturer. You don't want to overcorrect the PF of motors as this can result in overexcitation and overvoltages. The manufacturer can tell you the maximum capacitance required to correct for the magnetizing kvars.

you have to determine the actual kW of your unit, together with the actual pf of your induction motor... so that you can get the actual kVAr... but you can first check the entire power quality profile of your system, to also check for other losses like harmonics, unbalances, distortions, etc... before proceeding to actions...