Depends on the technology of the PC game in question and the libraries involved. A very old DOS game will take a lot of work to get going as the hardware API code would be platform dependent meaning you have to write your own hardware interface to draw graphics, play sound etc.
Games that are based on portable libraries like SDL etc are going to be much easier if they have already been ported to the target platform. All that will really need modification are controls.
There are also a ton of other factors that affect the port so the actual short answer is: It depends.
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