The Passphrase Cache is provided as a convenience so you need not type the same passwords over and over. When you decrypt a message, the program will check the cache and use the required password from there, if it exists. Set the cache timer to the number of minutes you'd like to keep passphrases in memory.
CAUTION: There are dangers in using the cache. If others might have access to your computer while you're away from your desk, they could decrypt your mail without knowing the passphrase. In a worst case scenario, don't use the cache at all. Set the timer to 0 minutes. If you use the cache, consider setting it for just a few minutes so it will be available while you're reading your mail, but then expire shortly after that.