There's no single answer. To get an estimate, you need to know the specific year and engine of your Neon and then check its EPA fuel economy estimates (city/highway MPG). Once you have those, multiply the combined MPG (or use either city or highway depending on your driving) by the size of your gas tank (which also varies by model year). This will give you an *approximate* range.
For example: A 2005 Dodge Neon with a 12-gallon tank and an EPA combined rating of 25 MPG might travel approximately 300 miles on a full tank (12 gallons * 25 mpg = 300 miles). However, this is just an estimate; actual range can vary based on driving conditions, driving style, and vehicle condition.