Should your 6th amendment rights have an expiration date?

Usually not. But if he was informed of his rights each time he was questioned, I that is usually ok. But the issue is a fine line since he was in prison, that is in a continuously coercive environment, I think the confession might have fallen under the rule. The rule stands for the proposition that you should not be tricked or coerced into making a confession without the right to have counsel present. It should be truly voluntary.