"How long am I responsible for her taxes to the IRS?"
If you are in doubt concerning your responsibilities or have serious questions as to whether you have fulfilled your obligations as personal representative, you should contact a tax/legal professional with experience in trust and estate matters.
Generally, your obligations are set forth in the publication linked to below. Regardless, the IRS, in most instances, has three years to audit a return (technically, assess a tax) after it is filed.
https://www.irs.gov/publications/p559#en_US_2018_publink100099488