Just over 1 year.
I run a very high-value, simple niche directory site that focuses on listing professionals in one industry by the US state they are located in. Its a small directory, and I don’t really make any money from it (just a few bucks more than what’s needed to cover the hosting and such), and as such I tend to only review the signups once a month or so at best.
Anyhow, I’ve had it running for about 14 months now, and from the start I had the occasional manual spam/unrelated/optimistic signups. They ranged between 10% and 30% of all signups. Traffic has remained relatively steady since roughly month 3 through month 14, as again, I really don’t work on this site hardly at all. Small traffic increases. Anyhow, I just now went to review signups and whereas before maybe I would have 30 signups in a month and 10-15 would be spam, this time I had about 400, with about 380 of them clearly being spam. It took 14 months, but apparently the directory is now on at least one if not more mass-spam lists.
I’ll be changing the URL and signup process in the near future 🙂