Continuous Improvement: How to know if the improvement achieved in our project is sufficient?

Throughout my career in Continuous Improvement I have participated directly and indirectly in hundreds of improvement projects.  All of them have reached an end state which is better than the previous, that is, processes have improved.  In each project I asked myself the following questions: Was improvement on this project good enough? Should I consider this particular project a successful one? Is it time to stop paying attention to this process and look for a new opportunity?  Well, for those who know me, my answer is always "it depends".  Let me explain.  It depends on several factors such as the current status of the process, the customer, the type of company and its expectations of stakeholders, etc.  However, generally speaking, I recommend considering the following points to indicate when the project has not achieved sufficient improvement, in order of importance:

1. When the committed target has not been reached: In most companies, regions, countries or cultures not meeting expectations creates an atmosphere of disappointment.  You must never take your eyes off what stakeholders expect from the project (including your own personal expectations).  Every company or department plans to use its resources based on clear expectations and our project is one of them.  For example, if we start a project that will reduce 15% of scrap in a manufacturing plant, this improvement represents a certain amount of money the company will save which, of course, is already considered for use in another area.  If your project does not meet the target you set at the beginning, then resources will be missing somewhere in the budget and disappointment ensues.  There is no point having driven a car with the family for 1,000 kilometers towards our vacation at Disneyworld if it is 1,100 kilometers away from home right?  How do explain that to the kids?
2. When there is still a significant gap vs the benchmark. Well yes I mentioned meeting targets is essential but, it does not mean so much if we know there are same or similar processes with infinitely greater performance than those achieved with the effort of our project. Even worse when those practices are within our own company.  For example, is not enough to reduce our delivery time to a customer by 70% or 80% if our competitor has a delivery time 90% shorter than our already improved process. Does it make sense to stop here?
3. When the cost of an additional improvement is null or minimum. Many we have before us the opportunity to further improve our process with no or minimal effort.  If this is the case let's go for it! The people who excel and get promotions are those that reach higher than expected performance, especially if it did not require additional resources.

It is important to recognize when we have reached sufficient amount of improvement since company-wide resources are limited--even in those who have staff dedicated full time to Continuous Improvement.  Recognizing that sufficient improvement has been reached will give us the chance to deliver an improved process back to the owner and focus on another opportunity that is waiting for us.