Most tax payers are eligible for and will automatically receive an extension as long as it is done on or before April 15; however, it must still be approved by the Internal Revenue Service. It is important to note that you are not "finished" until an acknowledgment has been received from the IRS stating that your extension has been accepted.
Now, an extension provides a taxpayer approximately 6-months additional time to file. When accepted a filer will have until October 15th of the filing year to file their return.
Taxpayers do not have the option to extend a tax return more than once. Under "normal circumstances" the IRS will not provide additional time to file beyond October 15th. Exceptions to this may be made for those fighting in combat, and in few cases, for Americans living outside the United States.
It is also important to note this will extend the amount of time you have to file but not pay any tax liability you may have.