Using a balance scale, what's the least number of times you need to use it in order to find the "false" coin?

If you are lucky, the least number of times would be ONE (if by chance you select the non-gold coin to compare with a gold coin, you will know immediately which one is lighter). I think your question wants to know the MAXIMUM number of times you would need to use the scale; in that case it would be FOUR (if by chance the first four pairs of coins you pick up and measure happen to be the 8 gold coins, which would obviously balance out) - which means the leftover coin would HAVE to be the non-gold one, no weighing necessary :)