By default, Enhanced Dictation waits 7 minutes (420 seconds) before quitting its processes. You can override this default by writing a different value to its preference file.
For example, if you want Dictation to wait 60 minutes (3600 seconds), open Terminal and execute the following command.
defaults write com.apple.inputmethod.ironwood DelaySecondsUntilTerminatationOffline -float 3600;killall DictationIM
If you want to return to the default value, you can remove the override using the command:
defaults remove com.apple.inputmethod.ironwood DelaySecondsUntilTerminatationOffline;killall DictationIM