What is the proper loudness for listening?

I've found that Paul McGowan has a point: especially with acoustic music (solo, unamplified voice; solo acoustic instruments or small ensembles; piano), the volume usually sounds best when it reproduces accurately the volume that instrument would produce in your environment. Therefore, even a piano can easily be played too loud; if your room is small, a grand piano would overwhelm it, and not just by taking up too much space. One of the things a good audio system should do is to accurately recreate the size of the instruments being played. Perceived size is not entirely a matter of SPL, but they are correlated.

When it comes to amplified music (rock, most jazz, electronic, etc.), this principle is harder to apply. As long as your amp isn't driven into clipping and the high frequencies still sound sweet, your pain threshold is about right for a band like Tool or Massive Attack. After all, that's where the volume would be in a live show.

Be aware, by the way, that "weighting" on SPL meters is crucial. Most are "A" weighted (note that the OSHA standard ditusa cites is given as "dBA"). Most laptop SPL meters I've seen, and most inexpensive hand-held meters on eBay or Amazon, are ONLY "A" weighted. What this means is that frequencies below 100 Hz are NOT registered at all! So if you're listening at, say, 70 dB with an "A" weighted SPL meter, you're probably hearing music that would measure 80-90 dB in "C" weighting, or even more. I learned this the hard way.

I like 70  to 85 dB.  Does not sound as good and feels pushed on me if it gets louder.  

When instrument volume sounds like what your hear it to be like in a live environment?  Except for rock concerts which may be detrimental to your hearing at the supposed actual volume.

Some approximations are below.

Generally, an acoustic guitar will put out between 70 and 90 decibels.

Piano (normal practice): 60 to 70 db. Piano (fortissimo): 84 to 103 db. Oboe: 90 to 94 db.

Cello ranges from 82-92 dB.

Violin ranges from 84-103 dB.

Chamber music in a small auditorium range from 75-85 dB.

The trumpet can range between 80 and 110 decibels, while the trombone can peak at around 115 decibels.

A drum set and cymbals is 119 dB loud on average but can range between 90 and 130dB (decibels) depending on what instruments are being played.

Generally, 75db is more than enough for me in my room, which is still on the way to being better treated.  Once it is treated properly, I may change my mind.

At what ever volume makes you happy, but as others have said, louder volumes can produce hearing damage, so if you do play at loud to very loud volumes take frequent breaks and keep your listening sessions short.

For me, between 60 and 70 db typically. 75 db is the most I ever want or need.