If you are due a refund and you do not file your return by the April 18th deadline you have three years to file and claim the refund due to you. There are no penalties, but the IRS could send you a letter asking you to file your return and if you refuse they could file the return on your behalf.
If you owe the IRS and do not file a return or an extension and you do not pay the amount due by the due date of the 2015 return, April 18th, the IRS can access penalties for failure to file a timely return and failure to pay.
