My own between wash wearing seems to vary with how long till our next laundry day. My wife, Valerie, and I tend to take our big piles of cleaning to the local Laundromat about once a month, whether needed or not. If things get desperate in between, we can always rush over to Wal-Mart and buy more shirts, dungarees, underwear, and socks. Family hand-me-downs, birthday or Christmas gifts, and thrift store offerings can come in handily as well, often in the nick of time.
Experiments have been done on how long a wearing of jeans without washing affects bacterial buildup. Oddly enough, even many months of more or less continuous unwashed wearing results in no more bacteria than a couple weeks.
acid-washed denim (Wikipedia)
A Google search reveals that the average for between washings wearing of jeans is 13 days, but this figure combines gender responses. If only men's habits are reviewed, it's definitely longer. "A month or two" were typical responses from the guys.
As with our skills as drivers and lovers, most people think they are above average in this regard but that other people are not quite as wonderful as they are. They are thus a bit more intolerant of others' tendencies to wear certain clothing items too long than of their own behavior.
What is a good way to tell if jeans really need to be washed? Well, if they have obvious stains (best not to ask of what, but one can imagine paint, sap, blood, oil, ink, grass clippings, chocolate, fecal matter, etc., depending on one's dominant activities) they may be getting close to their acceptable limit.
I think my wife prefers the nose test. However, this can be less than objective. We also think ourselves cleaner than average. Besides, if an article of clothing fails a nose test, the alternative is actually washing, not a desirable outcome. So we ourselves might find a "bit" of personal or jeans odor acceptable in our lower garments, even if less so for someone else's. Thus a solution may be to ask one's spouse, partner, or significant others(s) to "smell check" our jeans for us. If necessary their advice can always be ignored. In reality, though, this option is probably seldom used.
On his small Austin ranch, about 15 acres, my father used to wear pants for so long between washings that the crotch would tend to rip open, revealing too much as we were working on a new fenceline or whatever. That may be overdoing it just a little.