How late can a womans period be without being pregnant?

My significant other and I make love frequently. She doesn't use birth control pills, but I use condoms, and whenever we 'finish up' I am adamant about checking to make sure the condom worked as designed. (For her own comfort as well as my own.) Unfortunately, she is now a little over a week late, and she's beginning to panic. She has anxiety issues, has been dealing with a lot of work and college stress, and was recently sick. I've heard that stress and sickness can influence the regularity of the monthly gift. How late can a woman possibly be if she isn't pregnant?