It depends on the amount of iOS specific code and how your application is written in ObjC.
Our experience - a 3 months work in cocos-objc was transformed into cocos2dx in 6 weeks.
There are challages:
1. fix positions works differently
2. reading/writing of resources work differently on Android
3. for all iOS specialities (PDF, text2speech etc.) you need a wrapper or you find a C library working on both platforms
The third point was very challenging to us.