How much work is it for a developer to convert an iOS app into an Android app?

It really depends on the nature of the app being ported. If the functionality of the iOS app is straight forward an Android developer might not need to look at the source code of the iOS app which could save a lot of time. Also, it's worth noting that iOS and Android are not exactly the same.  They each have their own user user interface components and guidelines for using them.  Because of this the apps might differ slightly to take advantage of OS specific features and conventions.

100 % of the time that went into iOS development PLUS 30 % more (given that you'll have range of devices to test/ run compatibility and QA etc.). Moreover, you'll need another developer to develop on Android since finding a single developer equally expert on both technologies (and have time too!) is practically difficult.

It depend on the nature of app , first divide an app into three categories . 1) logic 2) UI 3) coding or syntax . If the current app has rich set of logic and algorithms then you can transform those algos into your android app without much headache. Second if your app a lot of design patterns which already available in iOS like navigation , tab bar etc which is not available in Android as easy as Interface builder provides , so implementing these widgets in Android can take alot of time . Third if your app has a alot of back end code which you need to transform into Android coding on top of java . So it takes some time to understand the syntax which way too complex then traditional Turbo C like code.

Look at http://MyAppConverter.Com , we offer online and automatic porting for Ios To Android with an average of 70% of coverage. We finish the work to have a fully 100% Android app for only 30% of your initial cost.