The idea is you do most of your wars early before the era's get too high. Either increased tech or Civic will increase your era.
From Industrial onwards you get 24 Warmongering points for a formal declaration... 1.5x for surprise. 50% of what ever you declare for each city you take... and I think its 3x city take for the last city....
So taking a city during a formal war 12 warmonger.... but if you for example declared a colonial war it would be 70% of that.
The length of time is used to discourage people from warring. If you liberate a Civ city you get -30 warmonger points I believe... it was changed in the latest patch from 36 if I remember.
Anyone that has denounced or is at war with the same enemy will get % reductions also.... so attacking the hated has value.
Bottom line... Warmonger points mean very little apart from immersion value and a bit of trade/tourism. The value of war far outweighs the warmonger points currently. Put on that what I said at the beginning of the thread and you realize war is worth it and peace is for those that want a longer game not based on the standard victory conditions.