how far apart should posted signs be placed?

For properties in New York, the NYSDEC says: "At least one sign must be set on each side of the protected area and on each side of all corners that can be reasonably identified. Signs shall be no more than 660 feet apart, close to or along the boundaries of the protected area. Since the signs must be conspicuous, they should be high enough, and spaced closely enough to be seen. Please don't turn your property into an eyesore by using more signs than are necessary."

The NYSDEC actually has 24 FAQs regarding posted signs, and can be found here.