What do you consider "loud" for your system?

When it blows the needle off the record.

I listen at 70db measured my with Radio Shack Sound Level meter for Blues music.  For Classical, it may be 3-4db higher to hear the bottom end of the dynamic range at a reasonable listening level.

My listening room has a lot of acoustical treatments so I get a nice clean, balanced sound.

Richmos, I listen to rock at 95 dB as measured (not guessing) all the time. That is a comfortably loud level and safe as long as you warm your ears up to it. With warmed up ears I will occasionally go louder. When I watch the Nine Inch Nails videos I'll push it to 100-105. If one can't listen at 95 dB comfortably it is most likely because something is distorting. Above 100 will disturb some people regardless of limited distortion like my wife.
n80 there are only basic rules. Like any other human trait there is a large range of sensitivity  to sound and ear durability. It is not just volume but also the type of sound. Impulse noise like a gun shot is more likely to damage hearing than a steady noise at the same volume. In terms of music, if you warm your ears up 95 dB will never cause damage unless you have really bad ears. Some people will go deaf even living in a monastery. Everybody's ears will accommodate to volume given a brief warm up. As the volume increases the smallest striated muscle in your body, the stapedius  tightens down on the stapes, the smallest bone in your body, dampening the volume your inner ear is exposed to.  Some people think that people who repetitively listen to loud music develop a stronger stapedius with a better damping effect so they can tolerate loud volumes better without damage. These studies have never been done because they would be very expensive and nobodies interested enough.
I have been to plenty of loud concerts and have had systems capable of at least 90 dB all of my adult life. As a teenager I could easily hear 20 kHz. I tested my hearing just last month and could just make out 18 kHz which is better than most 65 year old. Now that is anecdotal and everyone can not expect to do that. There is no accounting for luck. But, just because you like loud music does not mean you are surely destroying your ears. You can download test tones and check your own hearing. Plug one ear and run up the tones then plug the other ear and do it again. Our high frequency hearing always declines with age. I do not know the data for sure but I think most 65 year olds can't hear much above 12 kHz 

A person shouldn't present their personal anecdotal experience as a recommendation for the rest of the population. A few people have smoked a couple of packs of cigarettes a day and lived into their 90s, but that is hardly a foundation for telling others that smoking doesn't have a risk of lung cancer and other cardiopulmonary diseases. Most people suffer serious problems after years of smoking. You can substitute a number of other exposures in which the average person suffers damage while a few lucky ones don't.

The medical community and OSHA have good studies on the issue of how loud is too loud. NIOSH recommends that the average continuous exposure not exceed 85 dB and OHSA's PEL (permissible exposure limit) is 90 dB. As volume increases, the exposure time decreases -- OSHA says 100 dB exposure should not exceed 15 minutes. Most employers with loud work environments require employees to use hearing protection.

For my home listening these days, 80 to 85 dB is plenty loud for me. In fact, I rarely go to live rock concerts these days and use hearing plugs if I do attend one. I've even walked out of a few simply because the levels were so loud they were uncomfortable.

@mlsstl Thanks for that post. And yes, anecdotal experience is hardly a basis for personal choices. I can't tell you how many people I advise to quit smoking who have to tell me how Aunt Sophie smoked 6 packs a day, lived to be a 110 and was never sick a day in her life. The funny thing is that they often tell me this as they sit there wheezing from emphysema. The point being, folks often latch onto anecdotes that support what they want to believe and hold onto them even after the evidence (wheezing) has proven them wrong.

And while I think our perceptive tolerance to loud sounds can be attenuated to a degree, I doubt that 'warming up' our ears has any effect on attenuating the potential for damage.

I also disagree with this statement:

"If one can't listen at 95 dB comfortably it is most likely because something is distorting."

A pure tone at sufficient volume can become unpleasant and damage your hearing as well. Certainly distortion will make things worse, but I've got a pretty nice system and listening to even well recorded and well performed music above 95 dB gets old real quick. That's just me.

But, my apologies to the OP. He wasn't asking us for a lecture on hearing damage. I should not have brought it up.