"One website said 7 years"

This is more of a Practice Advice question. You would understand that record retention for the professional preparer is not the same as for the taxpayer. You provided a service to them in the line of your authority to file on their behalf, so your responsibilities are different than the taxpayer's personal responsibility. You might like to read this:

https://www.aicpa-cima.com/resources/article/document-retention-faqs-for-tax-practitioners

In general, you want to have whatever your client might need for as long as a client might need it. Beyond that, you need to provide for your own professional needs.