20 days payed vacation is standard in Europe. that's 5 days payed off per week, means 4 weeks off in total so 28 days away. This system wasn't though of for actual vacation, this system was though for shorter times away. And that is what people will use it for. wars being 3 days average if you have 4 players with low activity, ask them to take a time out for the sake of the kingdom. What monarch wouldn't pay 3 dollars to win a war?
Despite what you think of sitting, trading and all other methods used in utopia, the question remains the same: should you win a war based on activity and skill, or who has the wallet to cash out for his most active players to play double accounts?
It can be used in every war actually... it's a 3 day minimum sit time, and most wars aren't much longer then that, means you can use it on each account 7 wars per age.