When the SDK first became available, it often took around 2 to 4 weeks for experienced C and C++ programmers to get used to the strange Objective C syntax and conventions sufficient to do app size native coding, and maybe another couple months to learn enough about the iOS frameworks to throw together one of the earliest iOS Apps Store apps. However the bar of expected app features has gone up quite a bit, and the iOS API has gotten much larger over the past 7 years, so it would take longer today. On the other hand, Swift is said to be slightly easier to learn than Objective C.
If you complete the Stanford iOS course, including all the recommended pre-requisites, that's a bit over 1 year of university level coursework.
That's just to get started. But one can invest years of study and still not have learned all the iOS APIs, frameworks and system esoterica. Depends on what kind of app you want to develop. Simple or close to award winning.