when should you disavow a link?

I would never disavow anything. It's a mostly useless tool.
Google of course wants people to use it to identify crazy
webmasters and websites that fall for their spam identity
program. You now admit you are a "spammer," and you
have indeed good places to spam. Google has done
other things like this, as in nofollow.

Disavowing also serves the purpose of making people feel
good, and that they are doing something. Instead of just
ignoring the pesky fleas and gnats, they give them a useless
activity under the guise that it means something.

Of course I have heard official google people tell you that if
you are going to use the disavow tool, then you must do it
in the largest way possible. That is, disavow the hundreds, if
not thousands, of links. Mind you this makes no sense either,
as you would end up like a hamster on a wheel going nowhere,
but putting out a good, but wasted effort!

Just thought of something funny. Google thinks there are a ton
of webmasters caught like hamsters in a cage.

Think outside the cage.

Paul