config: add option for how long to wait until media ID expires

Author: sumnerevans

docs/usage/configuration/config_documentation.md

@@ -1805,6 +1805,16 @@ Example configuration:
 media_store_path: "DATADIR/media_store"
 ```
 ---
+### `unused_expiration_time`
+
+How long to wait in milliseconds before expiring created media IDs. Defaults to
+"24h"
+
+Example configuration:
+```yaml
+unused_expiration_time: "1h"
+```
+---
 ### `media_storage_providers`
 
 Media storage providers allow media to be stored in different

synapse/config/repository.py

@@ -141,6 +141,10 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
             "prevent_media_downloads_from", []
         )
 
+        self.unused_expiration_time = self.parse_duration(
+            config.get("unused_expiration_time", "24h")
+        )
+
         self.media_store_path = self.ensure_directory(
             config.get("media_store_path", "media_store")
         )