How long did it take you specifically to find your main champ(s)

Not long, fell in love instantly after seeing her being played by Doublelift back in S2.

how long should it take you to find your main champ(s)?

It's easier to main a champion that you enjoy than to main a champion for the sole purpose of winning. Preseason is the time to experiment with different champions and practice. You are pretty much still in the preseason stage. There is still much time before S7 ends.

Is changing mains a good idea?

Sure. If you no longer enjoy that champion, you can specialize in another. You can still maintain your skill on your old main while still learning and building knowledge/skill on another. It's only a concept to improve at the game more quickly and consistently really.

When should you look to change your main?

If you are playing purely for results, around 50 - 80 games if you can't get above a 55% winrate. If you are playing to enjoy your main champion; never.