My experience with Photoshop import filters is about six years, so take this comment with a grain of (old) salt.
I just took an example of import filter code from the official SDK as a template and added my own decode function. It was a direct task and took a week from start to finish.
If I remember correctly, the worst is:
Correct configuration of compiler options and compilation of sample code (two days)
Learn how Photoshop GUI works. Then it was not a standard win32, but something half based on the mac resource. I think they changed that, though.
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