I can't believe I'm on 150 days. Just more than a month left, then I've had these jeans on every day for 6 months, without a wash. I can hear some people saying "gross!", so I decided to look at that a bit.
Jeans smell, if you sweat when you have them on, and don't allow them to dry properly before you wear them again. Most people I know do not wash their jeans after every time they wear them, but wear between 3 to 5 pairs at any given time. This gives each pair of jeans a chance to dry properly, before they wear them again. It doesn't help if you're folding them up after you wear them and putting them back in your clothing cupboard, if they're not completely dry. This will lead to the germs, that cause the smell, to flourish, and that's "gross!". I wear this one pair all the time, and I take extra care to hang them up every night, and they're completely dry the next day. No smell, hence no "gross!".
The smell is not the only reason it may be "gross!". You also have to consider normal dirt, stains and such. Here there's no help other than dry-cleaning or just self cleaning them as much as possible when it happens. But so far I've kept them fairly clean, and completely smell free. Today however is day 150, and I think I should celebrate.