How many time it will take to conqure empty plot?

This will depend on your legion strength. Each enemy plot will have a durability of 500. Your legions will have a demolition value shown on the legions list, top left. If your legions demolition value is 250 then you need to hit twice to do the total 500 damage needed to take the plot. If your demo value is less then you will need to hit more.

To increase the demo value add more troops, higher tier troops. Use Footmen for the maximum demolition as they have a higher demo power.