IMHO, the right time to ask, is when you have a question and ask 
I am still a very much beginner. But the more songs I look through, it seem to be much more common, that there are chords borrowed from other keys, one way or the other. Also some songs change keys in the middle of a song and then go back again to the key the song started in. Some start in one key and end in another. No limits really. - Richard have made an extensive post about borrowed chords.
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