When you reach a certain level Scouts can take like what seems like 5 years and when you need 30 or 40+ provinces for an expansion it is just too stupid. There should be a way to add another Scout.
@Greg60 you will eventually reach the point when you will not get anymore Expansions for your city. As faster as you are scouting, as sooner you will reach that point.
Also, the costs for scouting increase as one progresses through the chapters. Plus, after all the nearby provinces are scouted and solved the encounters get harder. All mine are now 'hard' or 'very hard', with colossal negotiating costs. And as Solanix says, the city expansions will eventually dry up. No rush perhaps
My advice would be to scout all the time. However, after about halfway through the chapters, I only fought/negotiated the province encounters in order to complete quests during the events.