I'd ask them if it's ok to check with them every 6 mths, and I can't recall ever getting a No, so check back every 6 mths, you have to remember you don't want to come across as desperate. If you leave some info, and they show interest, then call back in a week, and find out there level of interest, if they (over the phone) kinda pass you off, ask again is it ok to check back every 6mths, this will secure your position as there next vendor, when they have problems with there current vendor.I have literally waited 3 years on some of my accounts, following this method, vending normally takes a back seat to everything, until there is a major problem, then you'd be surprised how important it becomes when the other vendor give poor service, be the first inline to secure that location when it becomes that major problem.
This is exactly the way this game is played. I would only add that you need to make sure that the information you leave looks professional. If it looks like a 3rd generation copy that came off an old copier, you won't do yourself any favors by leaving it.
You need to leave behind a well-assembled marketing piece, in color, and on flyer paper that you can buy at Staples or like store. If you don't know Powerpoint, then find someone who does and get them to help you lay one out. Use photos. Get their attention. If your leave-behind does not answer the question, "Why should we choose you as our next vendor?", then work on it until it does. And make sure you take it to Kinkos or Staples and have them print it for you using their color laser printers.