You should never submit more than once a month to the major search engines, Submitting more often could get you in the spammer area where your website will most probably be banned or penalized.

This is a common mistake that people submitting through submission software often do, Unaware of the fact that he is tempering with the engine.

Be careful especially with the open directory and Yahoo directory, I really don't know how google deals with such issues, but the open directory and Yahoo are strict in this sense.