However much you want!
You want to show your earnest desire to purchase the home. It's like putting your money where your mouth is. The more you are willing to put up front, the more you show the sellers your interest and ability to purchase the home. You are also showing how much you are willing to risk by the amount of money you are placing up with your offer. In our area (northern California), it is customary to submit an earnest money deposit of between 2% to 3% of the purchase offer.