How to tune the iTunes and App Store to remember the password for a very long time? By default iTunes and App Store are remembering your password only for 15 minutes and this is really annoying for someone that keeps the access to his computer safe anyway.

I am almost sure that these could me change by tuning a plist files from the command line. What is the secret command to make it -1 instead of 15? ;)
Enabling automatic downloads can help, but it's highly unlikely that this is simply a preference on the client. I would presume (but haven't reverse engineered) that the server is signaling the purchases that require a password either by signing a certificate with a short expiration time or upon each "purchase."