As an architect I would say under 150. As a player under 130.
I always think as a rough rule
MICRO less than 100
SHORT 101-150
MEDIUM 151-185
LONG 186-220
VERY LONG 221-250
I am in this boat, but as an archie, will say 150 back tees, 130 average tees, 110 senior tees, and under 90 for forward tees.
Niall, I recall having a course just miss the GD best new public list. In reading the comments, it was clear that the course, which porposely had all fun, mid length par 3's that a rating criteria was length differential in par 3's. So, for a while, I designed courses which had par 3's of roughly 130, 170, 210, and 250+ figuring no one would find those the same. (Back tee yardage)
I had always been taught, both from my mentors, and the super I worked under for summers in college, that average golfers just don't care for 200+ yards on a par 3. Even with top players now hitting much further, I probably split the tees further if I want the back tees to be extremely long. Because, who I am I designing for? Raters or players?
Add in the typical back ups at par 3 holes, and a slightly easier hole (rather than slightly harder one, as per Ross) makes some sense in avoiding long waits, and shorter holes are easier. Not to mention, public players still see par 3 holes as a breather and better opportunity for par than 4's and 5's.
I have never had the gumption to do a micro par 3 on a regulation course, though. I think many golfers would find that too easy, and maybe somewhat condescending, even if the hole isn't as easy as the length would indicate.