Many weeks. Some women for no good reason will skip periods even if they were like clockwork before. Stress, diet, and a variety of other factors can make periods late by anywhere from a few days to weeks to, in some cases, months. The only way to tell if she's pregnant is a pregnancy test. You can take those as early as a couple days before she expects her period, but like NJQT said, you can get a false negative in that case.

Her symptoms could be entirely explained by PMS. Your best bet is to wait until a day or two after she expects her period and take a urine test. Follow the directions on the package exactly. If that's negative, you should be okay. If she doesn't have her period in another week, take another test. But most likely if she isn't testing positive when her period is due, she's not pregnant.

Not true at all. Most women in their lives, at some point, will skip 1 or more periods entirely for no good reason. In fact, most gynecologists won't even want to see you about missing a period unless it's been over a couple months or so.