How moody is too moody?

I've a question and I'd appreciate input from males and females. My girlfriend could be described as moody. You can come home from work and she can be in tears about it. Or she can get in a temper doing some housework.

Now, this might all sound rather normal. I've been finding myself of late thinking that this is a permanent thing, and I'm not sure I'm very good at dealing with it. How do other people think of moodiness? How moody is too moody? How do others deal with their partner's mood swings?