One of the first questions I usually get when people come to me to port apps is ‘How long will it take’? A very rough estimate is the same order of magnitude it took to develop the iOS (or Android version). Obviously, you will have probably re-designed and re-implemented parts as you developed for the first platform. If you take off some time for such re-work then this will give you a very rough idea of the effort required for a port. Yes, Android and iOS have their quirks, testing and app review respectively, that can conspire to complicate timescales but on average, iOS and Android apps take of the order of the same effort. The projects I have seen that have had vastly different timescales per platform has been mainly due to differing developers having totally different personal approaches to development.
When porting either way, discuss with your developer how the screens might be represented on all the target screen sizes and orientations. For example, you might need to remove features or restrict orientations in some circumstances. These days, iOS also needs to use adaptive layouts so don’t start demanding or expecting pixel perfect layouts as used to be the case. Thinking things through beforehand will enable more accurate effort/time estimates.
Your developer will need to analyse what parts of the app need to be ported line by line and which parts are better ported by examining the intent of the app. Complex business logic tends to need to be ported line by line while os-specific things need new code that follows the intent/behaviour of the donor app.
When porting from Android to iOS, I recommend you read the App Review Guidelines carefully, even if you have previously published an iOS app. The guidelines have changed over time. I seem to spend considerable amounts of time explaining things to product owners who ask me to do things that won’t pass review. However, it obviously saves a lot of time not trying things that won’t ever pass!
There are more tips and hints in my post on Porting iOS to Android.