Let's see if I can help:
Is she even curious about her origins? If she knows she is adopted then I am sure she is curious.
What are the chances she wants to meet her birth family? If she knows she is adopted, I would say there is a pretty good chance that she would want to meet birth-family. She is the same age as I was when I decided to search - getting ready to go away to college, gaining freedom, etc.
At 18, is she even old enough to deal with the emotional aspect of a reunion? I don't think ANYONE is ever "old enough" to deal with reunion! That being said, she is at an emotionally-charged time in her life, with prom, graduation, college, etc. However, as she gets older life only gets busier and more complex, right?
Is there ever a “right time” to make contact? I don't know - maybe an "expert" on pyschology or reunion has a better answer than I do, but my answer is no. I think it happens when it happens for a reason - perhaps that is naive, but there it is.
How do most adoptive parents feel about reunions? You cannot lump all a-parents into one category to answer this. Some are supportive. Some adoptees do not share their search with their a-family. Some adoptive parents disown their child after they reunite with birth-family. It all depends. However, I imagine that it is an emotional experience no matter what the details, right?
Perhaps you could register with the state adoption registry where you live and see if she has also registered. If she has, there's the big green light to get in contact with her. You could also write a letter to her sharing all of your anxieties and emotions - and don't send it right away. If you do decide to contact her a letter might be less jarring than a phone call or showing up on her front porch.
I wish good things for you AND your bio-daughter.
Nicole