If you know one other language really well, Objective-C should take a week or two to get use to.
To understand all the design patters, apple HIG, XCode, profiling, libraries, debugging, app submission, etc, these combined is where youll sink your time to learn iOS development. Imo, Objective-C is the easy part.
I had 0 objective-C experience, but I made it work. It was a bit of a frustrating experience. Many times I found myself writing Objective-C boilerplate-ish code that I had 0 clue what it was doing, considering this is a hobby / for fun project I just wanted it working.
It's not easy to google the answer to, "Why did I just add this new keyword after this colon in this random .h file.."
I didn't want to spend the next month reading Objective-C for beginners, I know what a for loop is, I also know what constructors are. I just wanted to use the language.