[jacksond]

I am assuming you are talking about Android/iOS Native Apps. From my experience, the development time for these 2 platforms are essentially the same, assuming the same level of developer for each platform.

The amount of time needed can vary a lot depending on amount of 'polish' required as well how thorough the design and specs are.

With all of that said, I would give an above average developer 2-3 months to develop something like the Yelp app. This is assuming the back-end and web services are ready and available.