I agree - about 30 seconds per tune (like verse, chorus, quick fade). Highlight your best material (or best aspect) - obviously vocals sell any act. 4-6 songs tops (you won't keep their attention past that).
It's best to include some type of press kit (copies of any reviews, or write-ups from papers, trade mags, etc.). Band/artist photos can be a good tool (assuming your "look" is right for the club).
It's best to reach the club owner (or manager that handles booking) by phone in advance to confirm when they will be there. If at all possible, hand your marketing material directly to the person in charge (not a bartender). Follow up with a phone call a few days later.
With some clubs you may have to initiate contact several times before you can actually lock in a gig. Be patient and always be professional. As previously stated many clubs are booked 3-6 months in advance. The good news - once you get in a club (and assuming you play well and act professional) the odds are good that you will get into a rotation.
On a local level you can normally get a date (weekend ??) once a month. On a regional level (depending on your touring schedule) you may be able to book a date every three months or so.