How do I change how long my estimates stay valid?

Settings, Workspace, Estimates (/settings/workspace/estimates) has a Default expiration window, set in days from the day the estimate is created. Leave it blank and new estimates are created with no expiration date at all. A location can override it with its own window.

The expiration date is calculated once, when the estimate is created, and stored on the estimate. Two things follow.

Changing the workspace window does not move the date on any estimate that already exists. It changes what the next one gets.

Any single estimate can be given a different date in the estimate's own settings, and that date is what the customer sees. An estimate that has already been sent or viewed can also have its expiration extended without being rebuilt.

If your workspace runs more than one location, check the location before you change the workspace value. A location with its own window is where that project's estimates are actually getting their date, and editing the workspace default will do nothing for them. See how locations change your estimate defaults.

What else is on that page. The same Estimates page carries the default profit margin, the disclaimer printed at the bottom of every estimate PDF, the default subject and body of the email that goes out with an estimate, and the reminder schedule for estimates a customer has not answered. The margin is covered in how to set your default tax rate, overhead and margin; the disclaimer and the email in how to change what customers see on estimates and invoices.

Tax rate is not on this page. It sits on Settings, Workspace, Financials, because it applies to invoices and orders as well.

Changing any of these needs the "Manage workspace settings" permission. Without it the fields are visible but not editable.