on the first day of my first tour ever, i did 45 miles, hauling a trailer. man, did i think that was long! if i might be excused for tooting my own horn, a few months later on my third tour, i started a tradition which i still maintain on long tours: getting an early start and riding through the first night. on that tour i did 238 miles. since then i've never bothered with more than a rough estimate. of course, this doesn't exactly qualify as a "day".
what i think of as long or short is defined by time in the saddle.
a "ride" somewhere between 1 to 4 hours or so.
a "good ride" maybe between 5 to 8 or 9 hours.
a "long ride" is pretty much something that takes most or all of the day. or alternately when you're pretty much beat due to wind, heat, rain, traffic or whatever, and don't feel like getting back on the bike.