docs: document per-project preview expiration settings (#639)

Co-authored-by: mintlify[bot] <109931778+mintlify[bot]@users.noreply.github.com>

Author: mintlify[bot]

guides/developer/preview-projects.mdx

@@ -158,6 +158,41 @@ Promoting content allows you to sync charts and dashboards that you've created o
 
 Learn more about promoting content from preview projects [here](/guides/how-to-promote-content).
 
+## Preview expiration
+
+Preview projects are automatically deleted after they expire. By default, previews expire 720 hours (30 days) after they're created.
+
+Project admins can configure the default expiration and a maximum cap for every preview created from an upstream project.
+
+### Configure preview expiration
+
+<Info>
+  You need `update Project` permission (project admin or organization admin) to change these settings.
+</Info>
+
+1. Open the upstream project's **Project settings**.
+2. In the sidebar, select **Preview settings**.
+3. Set the values you want and click **Save**:
+   - **Default preview expiration (hours)** — used when a preview is created without an explicit `--expires-in` value.
+   - **Maximum preview expiration (hours)** — the upper bound. Any `--expires-in` value above this is silently capped to the maximum.
+
+Both values must be at least `1` hour, and the default must be less than or equal to the maximum.
+
+### How the limits are applied
+
+- **Previews created from this project** (CLI, GitHub Actions, or the Lightdash app) inherit the upstream project's default expiration when no value is provided, and any `--expires-in` value above the maximum is clamped down to the maximum.
+- **Standalone previews** (CLI previews with no upstream project) keep the platform default of 720 hours and are not clamped.
+
+For example, with a default of `48` and a maximum of `168`:
+
+```bash
+# Uses 48 hours (the project default)
+lightdash preview --name "PR: Add Revenue Metric"
+
+# Uses 168 hours (clamped down from 240)
+lightdash preview --name "Long-lived preview" --expires-in 240
+```
+
 ## Deleting a Preview Project
 
 Preview projects created using any method can be deleted in the [Lightdash App](/guides/developer/preview-projects#lightdash-app-2).

references/lightdash-cli.mdx

@@ -319,8 +319,8 @@ All standard [dbt options](#dbt-options) work with `lightdash preview`.
   - (default: 1)
   - Number of batches to upload in parallel when using batched deploy. Use with caution as higher values may overwhelm the server
 - `--expires-in <hours>`
-  - (default: 720, i.e. 30 days)
-  - Number of hours until the preview project auto-expires
+  - (default: the upstream project's configured default, or 720 if there is no upstream project)
+  - Number of hours until the preview project auto-expires. Values above the upstream project's [maximum preview expiration](/guides/developer/preview-projects#preview-expiration) are silently capped to the maximum.
 
 **Examples:**
 
@@ -382,8 +382,8 @@ All standard [dbt options](#dbt-options) work with `lightdash start-preview`.
   - (default: 1)
   - Number of batches to upload in parallel when using batched deploy. Use with caution as higher values may overwhelm the server
 - `--expires-in <hours>`
-  - (default: 720, i.e. 30 days)
-  - Number of hours until the preview project auto-expires
+  - (default: the upstream project's configured default, or 720 if there is no upstream project)
+  - Number of hours until the preview project auto-expires. Values above the upstream project's [maximum preview expiration](/guides/developer/preview-projects#preview-expiration) are silently capped to the maximum.
 
 
 ### `lightdash stop-preview`