"When you think it'll help" is probably the best answer I can give to this one.

As much as I'd love to say all the time, there comes a time when some people expect you to care about something that doesn't really deserve it or need it. Caring about people, the environment, and the future is important, but fussing over the little, materialistic things just isn't really needed.

Also, there comes times when the best approach is to not care what other people think, especially if they're needlessly critical, have doubts in you, and negative in the first place, or are simply selfishly starving for your attention needlessly.