Applying again after turning down job offer

BLUF: After turning down a job offer, is it promising to apply again to an orgaization?  After how long?  What is the best approach?

This spring, I was in the fortunate position of receiving exciting job offers from two great organizations (thanks, Career tools).  I would have absolutely loved either offer.

Both organizations are fairly large one-city companies (200-400 employees), and I talked a fair amount with the CEOs of both companies, as well as many other staff members.

Ultimately, I turned down one of the job offers.  I let them know by phone and by e-mail how much I appreciated their time and their offer and regretted not being able to join their team.  I had gone through 8 rounds of interviews with this organization over the course of 3 months.  I've stayed in touch with several members of the team there.  Our companies collaborate closely and have a great working relationship.

I am happy with the job offer that I accepted, but I can see some situations in which I may be interested in making a change, and my top choice would certainly be the other company.  How many years would I need to wait before resuming conversations with them?