Designing and building a mobile app is similar to the pre-production and production stage of a television/film project. If you hire a novice writer or production manager, you'll end up rewriting the script over and over or have to deal with cumbersome logistics that will incur more production costs.
A great mobile app has to be built with robust code and architected well, and this only comes with experienced developers.
Below is a sample of two ends of the pole on how long it takes to build quality mobile apps.
Simple App : 1-2 weeks to 2 months
Complex 3D Game App with animation and models : 4 to 6 months