There is no formal table, because it always depends on circumstance. You may take 20 whenever you are not rushed, and there is no penalty for failure.
About the only absolute statement you can make is that the skill must allow retries, since taking 20 represents making repeated tries until you succeed. The Appraise skill, for instance, explicitly states that no retries are allowed, so you cannot take 20 on any Appraise check.