I'm a dev at a company and I pushed for us to have a staging environment _and_ QA. When I started at my current job, it was common practice to edit code in production, and there was no testing (automated, QA, anything).
In our case, having a staging server (which is really a prod server since it's used for some internal stuff) gives us a chance to check a few things:
1. The installer still installs everything properly.
2. The main website works
3. Web services work
We don't have any hard and fast rules about how long things sit on staging, but having a staging system is useful.
So there you have it, a staging environment driven by engineering.
In our case, having a staging server (which is really a prod server since it's used for some internal stuff) gives us a chance to check a few things:
1. The installer still installs everything properly. 2. The main website works 3. Web services work
We don't have any hard and fast rules about how long things sit on staging, but having a staging system is useful.
So there you have it, a staging environment driven by engineering.