Q: How long does a revised paper remain at the 'Required reviews completed' status?

When the status shows "Required reviews completed," it means that the reviews have come in. While it is generally uncommon for the peer review to be completed within 6 days, since this is a revised manuscript, the reviewers were probably already familiar with it, so they could review it within a short time.

However, because the reviews were usually quick, it does not necessarily mean that the decision will also be quick. Generally, journals receive a large number of submissions and the editor has to handle multiple submissions. It is possible that your paper has not yet gone to the editor, since there are other papers that have come in before yours. Your paper is possibly still waiting to be checked by the editor. Once the editor starts checking it, the status will change to "With editor" or "Evaluating recommendation."

This process can sometimes take several weeks. So, don't lose patience; if you do not see any status change over the next month, you can send a polite inquiry.

We have a handbook on templates for communicating with the journal which includes a template for inquiring about the status of a paper. 

Related reading:

Why is my manuscript's status not changing from "Required reviews completed"?