I LOVE These Philosophical Threads
There is too much to consider to set a specific distance as adequate input for a par assignaton of 6, or any other number, from 2 up. Distance is perhaps the primary consideration, but other factors, such as effective length; foliage density; fairway direction or shape; OB's, Mando's and/or other obstacles; topography and terrain; and especially the skill level targeted are as or more important. Overall course par schematic should also be considered. But maybe the most important question to answer in designing a hole to be a Par 6 or not, is "What does the land give you?"
In the original 1000'+ Blue/1200'+ Gold design of #8 at Springwood, it was done for rarity, overall par schematic along with the two par 2's and because it was what the land presented. My biggest misconception of that hole was that there was going to be a local players' association willing to do the work to make the hole work, which there never was and therefore, which it never did.
As mentioned above, the PDGA does present guidelines. I've been informed that I give too much credence and credibility to that chart, as it is antiquated and obsolete and should be trumped by the personal opinions and preferences of players of youth and high skill level, even (and perhaps especially) when they have no real design experience, talent or vision.
When we were designing Johnson Street in the early, early 90's, I proposed a 777' long hole for the power tower field. Such was unfathomable at that time, as the longest drives by the best players were in the 250' range and there were only Par 3 holes in existance. Now, twenty-some years later, mostly due to plastic improvements, that distance in that type of wide openness is considered a Par 5 for Red, White and Blue tees and a long Par 4 for Gold level players.
As a final and perhaps repetitive point, relative to the brief Par 2 discussion above...ALL holes are filler holes, in that they all fill the space between the preceding hole and the following hole.