Rachel McQueen, Ph.D., is an associate professor of textile science at the University of Alberta, Canada.
You’re wise to not wash denim too often. Laundering can make clothing deteriorate faster, particularly with cotton. Microflora (i.e., bacteria) can be transferred from skin to clothes, but I wouldn’t worry about those microbes—the bulk of them came from your skin, so they’re not harmful.
The worst side effect of not cleaning jeans is odor, so use your nose to tell you when it’s time to toss them in the machine—some people can go six months or more; others, a couple of weeks. When you’re between washes, hang them in your closet instead of in a drawer—airing them out keeps mustiness at bay.