You have 3 years to file a return if you are expecting a refund. If you don't file by then, the U.S. Treasury simply keeps your "donation."
If you owe taxes, it's recommended to file as soon as possible. Information about late filing can be seen in the following FAQ. https://ttlc.intuit.com/replies/4159821
You can file a 2015, 2014, or 2013 tax return using the TurboTax CD/Download software. See the following FAQ for more information. https://ttlc.intuit.com/replies/3301770