Yep, this is probably another stupid question...

You can definitely google this. But it varies by genre. Fantasy and sci-fi are a little longer because of world building.

As long as it takes to get from "the beginning" to "the end".

Are you worried about being too short or too long? What genre within YA are you writing? How complex is your main plot? Your subplot(s)? Does your book take place in our world, a modified version of our world, or somewhere else entirely?

I've seen everything from 40k-ish up to 100k depending on the needs of the book in question. If you sit down and really think about what kind of a book you are writing, your next step could be reading other YA books in the genre that your story fits.

I don't think, for the most part, that there are arbitrary magic numbers that spell success or doom. There are stories and the words that it takes to tell them well. It's the 'telling them well' part that ultimately makes all of the difference.

Sorry to be so abstract, but there really isn't a completely solid answer to your question.

40,000 is quite short. Any less and it wouldn't qualify as a novel.
But you shouldn't add anything you don't need. You could consider a subplot.

Once your novel is finished, you might also consider some betas. If it feels like it's missing something, they'll be able to point it out.

I always aim for 6, but that's because 3 is a magic number and 30k is too short.

Here's the link that's almost always referred to when word count questions come up: http://theswivet.blogspot.com/2008/03/on-word-counts-and-novel-length.html

45k-80k for YA, 75k-90k for paranormal mysteries. These are just estimates, of course.

Here's another good link: http://literaticat.blogspot.com/2011/05/wordcount-dracula.html