The actual Zelda 3 was in development for the NES then cancelled.
The png file on the page of the same data is 741 bytes. An uncompressed 4-bit bmp of the same data is 2250 bytes.
To answer the initial questions, I remember that the compression ended with like 40-20% fewer bytes for things without really big empty space. Later games on the SNES used lzss like compressions.
Thou my owns implementation of the compression give better results (most of the time) for the original datas of the game that the tools used by Nintendo and it's pretty naive.
So the amount of compression really depends on how much the designer is willing to compromise their vision.
I would guess that the average amount of compression might be better than 4:1. (That is less than a quarter of the uncompressed size)