You have 3 years to claim a refund, from the date it is due. 1
So if you didn't file your 2015 taxes yet,
but you did request an automatic extension for your 2015 return,
you can file and get your 2015 refund up until Aug 15, 2019 (instead of today April 15).
There is also another extension you can file that extends the due date until October 15, but I think that has to be done by a tax preparer.
Remember the topic is refunds... if you owe the IRS today (15-APR-2019) you have to pay what you owe by today. If you file an automatic extension it moves the due date of the paperwork (the filing)... it does not move the date you owe any money which you may owe.
This is not intended as tax advice, it's just ramblings from my personal experience.
1 This could be off a few days, see first comment.