what kind of camera do you have?
how di you find out its 1 frame every 15 mins, just trial and error?
I've shot with a variety (and combination) of cameras--DCR-PC100, DCR-HC90, HDR-HC1, and even a PDW530. My current set is 2 HDR-FX1s and 1 HDR-FX7. They're all able to stay in-sync with each other down to the frame for 2+ hours at a time during free-run. I've yet to find an audio source that's that accurate, but I have high hopes for the 744T that's coming tomorrow!
And yes, I found 1frame/15min by syncing to a drum beat on the video's soundtrack. I'm not saying that's a perfect scaling factor, but it's still quite good after 3+ hours, and that's good enough for me.
Oh, and needless to say, this is all open video/taping with the band's knowledge and venue's permission. Three cameras don't exactly go unnoticed, especially once the tripods come out!