What is the proper loudness for listening?

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.

I have always asked the question if one can measure dynamics by a decibel number. There is a certain point in volume that makes the music come alive. You can hear the instrument separation, you can feel the kick drum punch you in the chest, and you can listen for hours with no fatigue. Yes the room, the room treatment, and the components will all play their part but I still think there is a low and high range you can measure that will always work and achieve that sweet dynamic sound stage or nirvana. 

Yes. Another way to say it is the optimal level is the loudest you can go for extended periods without risk of damaging hearing. Dynamic peaks in the mid to upper 80s db level starts to breach into that territory. A sound meter app like decibel on iPhone is your friend! When you start peaking in the yellow zone you are where you want to be in most cases ….. beyond the green and short of red.

optimal level totally depends on the situation... including other people in the room, your purpose, time of day, are you engaged in other activities, etc. ... and depends on the speakers, etc., too.

some speakers need to be "turned up" to come alive... I'd never want such speakers.  Others do very well at even quiet volumes; I'm constantly impressed by the dynamism and richness coming from the Klipsch Heresy at very low volumes, for example, and some of the most interesting listening experiences I've had with the Epi 100 over the years have been at low volumes late at night: they have clarity and sparkle.