Remember, lowering the bike will affect handling and ground clearance when driving straight and leaned over.
As far as how low, the 3 hole lowering liks usually give stock, 1" and 3" drops. Up front, you can slide the fork tubes through an aftermarket clamp and play with the ride height. Our bikes look awesome slammed to the ground, but it's not practical riding like that. If you're lowering just for looks, try 1" to start and see how you like the ride/handling.