Not very long, that's for sure. I now think that our next visit to DL after opening week of the HM was less than a month later, because it was part of a church youth group outing, and that kind of thing would have been scheduled as a last summer blast before school began in September.

Keep in mind that what you see in the above post represents the sum total of evidence for the existence of this original effect. Since no one can objectively test my memory, not even me, the case rests entirely on whether the correspondence between (1) my posted description and (2) what is pictured in the blueprint is too much for coincidence.

Actually, I should mention that there is one other teeny tiny shred of evidence: Marc Davis's original concept art for the HHG shows them floating along with wispy trails. See page 88 in Surrell's The Haunted Mansion: From the Magic Kingdom to the Movies.