one free - but complicated solution - would be Apt,
http://www.eskimo.com/~jibanes/apt2.zip
User guide:
Apt for PalmOS
Its a program that allows you to set the turn-off timing (and a million other things) on a per-app basis.
Its a tinkerer's dream, but not designed for quick intuitive use.
If you do decide to use it: Having chosen your Flashlight app in top right dropdown list, Set AutoOff to "0" using the slider bar (it's an option in the Prefs tab, which should appear by default when you start Apt).
In the first screenshot on this page
Apt for PalmOS: Quick Tour
AutoOff has been set to 0 for the app "Contacts"