How to remove leftover files after an application download?
You are completely right with your assumption: Whenever an app is installed, it is first copied to /data/local. You can check that directory if either you've got a file manager permitting you to go there, or have the Android SDK installed. No root needed, as this directory is readable and writable for the shell user.

But to your last question: No, I'm not aware of any mechanism to control that delay (never saw a corresponding setting). If it can be configured (and is not hardcoded in some place), this will most likely involve root privileges.