Furthermore, when do ideas become common enough that it is Ok to use them?
I have two answers for you:A)
under U.S. copyright law, ideas are never
protectible--only expression is protected. Copying art work that someone else has made is not
copying ideas, however--it is copying expression. ("Style," on the other hand, falls on the idea side of the line.)B)
You seem to be trying to talk about things that are in the public domain. Basically, you can copy expression with impunity once the copyright runs out, because the work then enters the public domain. Right now (don't quote me on this) I believe it lasts for the life of the author plus 70 years. By contrast, it's worth noting that the courts come down especially hard on people that violate the copyright of works that are still unreleased! (Here is
the Supreme Court smacking around somebody that published a portion of ex-president Ford's unreleased memoirs without his permission.)