Usually just a couple of months. I found one brand that worked for me, and it was always the plug or wire into the earbuds or the y split shorting out somewhere, and way before that point was reached the cord would lose it's fexibility and harden and have a ton of microphonics.
So I then figured out how to open them up, and bought and experimented with cords from cheap earbuds. That was ok for awhile, with cords lasting form 3months to a year, but now I finally found a good wire that was purpose built to be a high quality wire, for the same price as a cheap set of earbuds.
That wire/plug I'm still using after 3 years of 24hr/day/365 use.
Now my issue is earwax vapor rot of the actual drivers themselves. This can go anywhere from 3 months or 2 years. One day the sound will be different, or short out in a weird way. Nothing I can do about that, so I'm searching for tiny replacement drivers. Im in a bit of mourning. The earbud I used revised it's construction and now use drivers more boomy and less flat, so I'm looking for new drivers that fit these tiny shells. If anyone knows where to source decent drivers that are 6mm in size that'd be great. So far I've found some really nice drivers from knowles but with shipping they are $40 for a single experimental pair which is too much. My earbud that I use costs $30, and I'd get a spare set of shells and a cable with a decent right angle plug that I can repurpose for something else.
It's been an interesting journey. I never realized how cheap parts really are. Asides from the shells, which I know how to make now, a really nice BA driver can be had for $10, or you can go chinese for $5, and my replacement cable, that I found for $6 is miles ahead of the cables that I find on bose or beats earbuds. I'm really starting to consider making my own earbud company, If I like my own custom build, and I hate the shortcomings of most earbuds I see out there, there must be someone else out there who's looking for the same.