Assuming that you have not done any source code modifications to the SDK itself and have built your game from scripts, then it's simply a matter of letting Unreal prepare the packages for the different platform.
Note that this is if Unreal supports the platform you're porting to, if not then you'll obviously have to modify the code that packages the data for runtime use and write new low level hardware handlers.
Building a game that works with different control inputs (keyboard vs. gamepad for example) and designing a GUI will be where most of your work time programming is spent after the hardware support is decided. That schedule is no different for Unreal than any other port, you can do a cheesy job fast or a proper redesign for the platform depending on your needs.
Finally you will have to handle the new platform's TRC (Technical Requirements Checklist) needs and submission+approval needs and it would be a big mistake to not factor in that additional work.
Summary: "how much work" and risk analysis are entirely dependent on your dev team and source project quality so I tried to give broad strokes to cover the main issues - but don't have a real answer to your specific question.