@Sho-dan the Benevolent welcome to the forums
The city shield only prevents attacking, not plundering
Once a attack has been successful a player has 24 hrs from the time of attacking to plunder a single item.
My recommendation would be to only buy the 48 and 72 hr timers as those guarantees a minimum of a day's rest plundering
Attackers are already on a 24 hr timer. So by putting a same length timer it'll only be worth the difference in time between when they last attacked and when you put the shield up. If they attacked 5 mins ago then putting the shield up straight away will only stop them for 5 mins as they were already locked out from attacking again for the rest of that 24hrs
It'd prevent additional plunderers in that 24 hrs sure because if they can't attack they can't plunder. But unless you're in a particularly bad hood then you might only be dealing with a single guy anyway