I am a former IRS agent and teaching instructor and I wish I had a dollar for every time somebody asked me this question.
Let me answer this question right off the bat.
There is no statute of limitations on back tax returns.
IRS has the right to ask you to file as many back tax returns if you did not file your tax returns.
As a Practical Matter
As a practical matter however, the Internal Revenue Service will ask you to file the last six tax returns to get your case back into the system. In some cases, the Internal Revenue Service may require the last three years worth of back tax returns. Each case is worked individually based on the facts of the case.
It is up to the agent  on the case to make that decision.  Many times your assets will play a factor into how many back tax returns the IRS will determine you need to file.
Sadly, many taxpayers lose there tax refunds and tax credits by failing to file back tax returns.  They also lose Social Security credits as well.
A good professional tax firm can help you through the process and save you many tax dollars by understanding the system, the protocol and what it will take to satisfy IRS without it costing you a fortune.
The Statute of Limitations for the IRS on back tax returns.
The code does not address this because there is no statute, however  Title 26 of the United States Code, Section 6502, sets a statute of limitations for the IRS on the collection of unpaid taxes.
Many taxpayers wrongly interpret this code section by assuming that if an unfiled or past due return occurred more than ten years ago, the IRS cannot collect taxes.
The statute of limitations applies once taxes are actually assessed, either by means of a return that is filed late, or one that is prepared for you by the IRS, or any settlement agreement that you have come to with the IRS.
As a side note, that collection statute runs from the IRS date of assessment and not the date filed.
There is another statute of limitations that precludes you from receiving a tax refund for an unfiled return that is more than three years delinquent. If you think you have a tax refund coming in better file your tax returns within three years  from the  April 15 due date.
Once your unfiled returns passes this three-year mark any tax refund that you could have received had you filed in a timely manner becomes the property of the IRS and Uncle Sam.  Unfortunately many taxpayers have the ostrich syndrome, they placed their heads in the sand without realizing they lost their tax refunds and tax credits.
The Tolling the Statute of Limitation on Collections of Tax
The 10-year statute of limitations can be extended  in some circumstances.
This occurs when you file a appeal with the IRS, such as a request for relief (a ( CDP ), an offer in compromise,  the filing bankruptcy, tax litigation can also toll the statute of limitations, as can signing a waiver that allows tax collection to extend past the 10-year mark. This waiver is on IRS Form 900. You should never sign a waiver unless you get professional tax advice.
IRS can prepare your tax return under 6020 of the IRC. They can file what is known as a substitute for return, a SRF.

If you do not file your returns, the IRS will prepare a substitute for return for you based on an estimation of your income. IRS can simply reconstruct your tax return or they will prepare your tax return based on information they have on their CADE 2 computer  system based on third-party records. Any time anyone has sent income to you over $600 they are required to send the IRS W-2s, 1099 or other documents. IRS keeps these on their system for seven years.
The Internal Revenue Service will prepare your tax return allowing you as a dependent and that’s it. They will allow you no business expenses, no tax credits, no miscellaneous the deduction’s.
You will pay the highest amount of tax by law if the Internal Revenue Service prepared your tax return based on SRF. So if you do not file for a number of years the IRS will simply file your tax return.
If IRS filed your tax return for you you can reverse this tax process by filing for an IRS audit reconsideration. Contact us for details on this procedure.
If the IRS makes a tax assessment for you this will cost you dearly

The Internal Revenue Service generally prepares tax returns  for taxpayers that they know will owe tax. I have never experienced the IRS prepare tax returns for people and issue refunds.
Once your back taxes are assessed, the IRS will send you a Notice and Demand for Payment, after which you have ten days to respond. As a general rule they will send you a series of 3 to 4 notices. Each one stepping it up a little more until they’re ready to put the hammer down.
Should you ignore this tax notice, the IRS will then send you a Final Notice of Intent to Levy and Notice of Your Right to A Hearing.
An IRS agent may deliver this to you in person or leave it at your home or place of employment or send it by certified mail.
If the IRS cannot locate you, it will mail the notice to your last known address by certified or registered mail. Unfortunately many taxpayers that have moved never notified IRS in will receive a bank levy or wage Levy garnishment without ever receiving a tax bill from the Internal Revenue Service.
Once the IRS sends out its final notice and you do not call the Internal Revenue Service you can expect IRS to take enforced collection actions.
Collection methods of the IRS
The IRS has a variety of methods in its arsenal. They are the largest and most powerful collection agency in the world.
The IRS can place a federal tax lien against your assets for the amount of the unpaid taxes.
As a general rule the Internal Revenue Service will issue an IRS Bank Levy or wage garnishment notice.
If you received an IRS bank levy you have 21 days to reverse the process by calling the Internal Revenue Service.
If you are received an IRS wage garnishment notice the employer is required to send your next pay check to the IRS. Some exemptions are available for an IRS wage garnishment.
Should you have any questions contact us today. We are available for free tax consultations. We are the affordable nationwide tax firm.
 
How many years of Back Tax Returns should you file – The IRS Statute of Limitations