Our first experience was an unplanned, spontaneous instance of same room (tent) sex with another couple on a camping trip back before we were even married, in our college years. We had never discussed anything of the sort before then, so if that counts as "swinging" then there was about 5 minutes between when the idea came up to when we jumped right in!
The next morning we all talked about what had happened, and decided we were going to make it more interesting that night, and added a little FF action to the mix, with a little light touching between couples. I had no idea what that was at the time, but after my terminology education from this site, I believe that would be classified as a "soft swap".
So in that case, it would have been a little less than 24 hours before it was first brought up, to first encounter