Sometimes there is a requirement by a state regulatory entity (usually the state Motor Vehicle Department) for an insurance company to certify that a driver has the ability to pay future claims up to the state required limit. This is accomplished by an SR-22 insurance policy.
The typical length you are required to keep an SR-22 is 3 years, but states and judges can extend or shorten the length requirements.
Non-owner insurance policies allow the named insured to drive any vehicle they are given permission to drive by the vehicle owner. If you put the two together it is called a non-owners SR-22 insurance policy.
Keep in mind that non-owner insurance coverage always equates to an excess of the liability coverage originally purchased by the owner of the vehicle. In other words, if you have an accident the owner's liability insurance pays initially, until the amount reaches the limit of the policy. A non-owners policy does not equate to coverage for the vehicle being driven by the insured.
Named, non-owner insurance coverage invokes when the cost of injuries and/or property damage exceeds the limit of the vehicle owner's liability policy. An SR-22 policy is typically required for three years. A Non-owners SR-22 insurance policy does not extend to other vehicles in the insured's household. This has something to do with the law and could be better answered by an attorney.
You can obtain a free quote for an SR-22 policy here.