Comment 9 for bug 132456

According to my (and others) tests, the password isn't stored indefinitely....
Just 15 minutes as sudo will not prompt for the password in that period.
I tried launching kdesudo commands within the shell every 5 minutes, and also within the kmenu using adept, after the standard 15 minutes caching, I'm prompted again for the password in any case...

You may have an issue with sudo on that point, as the caching is managed by sudo and sudo only, kdesudo doesn't have any mecanism to store passwords, as it is just (once again) a wrapper for sudo commands.
I asked other people to test this too, and all confirmed me that the password indeed isn't stored indefinitely...
Hard to figure out what happens since I can't reproduce....