Expect delays and remain flexible in your timeline expectations: “Unforeseen circumstances like new iOS releases or third party integration changes, or unexpected problems that arise during quality testing might extend your timeline. Give yourself a buffer rather than a firm deadline to complete your app. The last thing you want is to release a product to the App Store that’s broken, buggy or worse. You only get one chance to make a first impression, and you don’t want to be one of those apps that is downloaded and immediately discarded” (Sarah Riley).
How Long Does It Take to Develop a Mobile App?
Josiah Humphrey

Too many organizations don’t consider the delays. This is a great point.