Are you referring to completing the Settlement chain within the timeframes that will earn you bonus rewards or just finish it period?
While I have not come close to finishing within the Bonus Reward timeframes, I have completed the Viking and Japanese Settlements multiple time (though not the Egyptian one, though I've tried only once). I think my personal record for completing the Japanese chain (which is the one that appeals to my playstyle) is about three weeks, so, obviously, Settlements are not a priority for me. That said, while I might be glacially slow, I've also not spent a single Diamond in the course of completion.