How to calculate run time of inverter

I have a 24volt inverter (2kva). There are two 12V, 200AH AGM (C20 at 27 degree c)batteries connected in series to the inverter input. Now, if I have a constant load of 2.5 amperes at 120V, how do I calculate the run time of the inverter for 70% DOD, 50% DOD, 30% DOD, 100% DOD ? Is there some forumula ? The power factor of the inverter is 0.8 and efficiency is 0.92. Also, how much time would it take to charge the batteries (40-50 amps charging current) back to 100% from 20%,30%,50%,70% of charge left ? I have a lead acid charger which can charge upto 50 amps.

The battery manufacturer says:
Charge Regime:
Batteries shall be charged in constant potential mode with current limit. Float charging voltage: 13.5 volts per module Boost charging voltage: 13.8 volts per module Current limit: 20.0 Amps to 50.0 Amps

 

Thank you.
Also, how much time would it take to charge the batteries (40-50 amps charging current) back to 100% from 20%,30%,50%,70% of charge left ? I have a lead acid charger which can charge upto 50 amps.

 

I understand that I can charge a 200AH battery with a 50 amps charger in 4 hours. But, with the AGM batteries there is bulk and float charging, right ? So, my question is how many hours would it usually take with both the bulk and float charging stages ?