10 years songwriting, 8 years digital composure, almost 2 years mixing and mastering.
I have remade a few song in my time, and know how to get this job done within several hours.
If the song is complex, with plenty of frequencies and instruments, then it may take up to 3 days to finish completely ( built, mixed and mastered).
I am guessing that you do not need the song to be mixed or mastered though.
Just a partial mix will do.