There is an awesome main building (Yggdrasil) that you get after finishing the Settlement once. Then, as you finish it more times you get Emissaries for your Town Hall that provide various bonuses, such as troops or goods and such, alternating with upgrades for the main building. Once you have all the main building upgrades and Emissaries you get 50 FP every time you complete the Settlement if you continue with it. Every Settlement has a main building which is the reason to complete it. I only do the Vikings and Aztec Settlements, because they're the only main buildings that provide attacking army boosts.
You don't have to pay attention to the timer, it only has to do with getting fragments of a secondary building that, frankly, isn't worth the bother.