There's no "standard." It really varies, based both on the value of the property AND your local norms and expectations. In some areas, $500 is OK. In other areas, it might be $1,000. Or it might be roughly 1% of the purchase price of the home.
The larger it is, generally, the more seriously the seller will consider your offer. An earnest money deposit of $10,000 on a $300,000 home will be taken more seriously than an EMD of $500. Just make sure your offer is written so that you can get your money back if circumstances change.