Glad to hear that you are enjoying Yin Yoga.
Your question of "how often" is a good one: it has been discussed before in the forum. There is a more complete answer at this post: http://www.yinyoga.com/forums/viewtopic.php?t=410
. You may want to read it and check out some of the research cited there.
In short: it is generally safe to do Yin Yoga daily, but, as always, it depends on the student. Nothing is okay for everybody. You have to listen your body between classes and make sure you feel it is okay to keep going at the current pace.
The idea that more frequent but shorter sessions might be as good or even better than longer, fewer ones rings true to me. I do Yin Yoga every day but not always in a formal "yoga practice". Sitting on the floor as you read, or watch tv or talk on the phone is yin. You can read quite well while in Swan. You can eat meals while sitting in Half Butterfly at your coffee table. You can watch t.v. while in Sphinx. All this is yin, without the formality of a yoga class. I haven't found too much problem from daily living on the floor.
Of course sitting in a chair for 8 hours a day is also yin: you can do too much yin! Studies have shown that how we sit is not the real problem, it is how long we sit in any one position. If we sit slouched for 20 minutes or more at a time, several times a day, the lower back is going to suffer. If we sit ramrod straight for hours at a time too, the back will suffer. The key is variety: to move. Sit one way for 5 minutes or so, then change to another way for 5 minutes or so, then switch again.
Let me know if this doesn't address your question.