In the dispute between the FBI and Apple over obtaining access to the terrorist's cellphone data I thought of the concept in my title. Radioactive elements have a half-life, the amount of time it takes any mass of the element to emit radiation and convert to half the mass. (Not a good definition but look up wikipedia if you want better.) The point being each element decays at a set rate, fixed by nuclear physics.
Apply the same concept to the information of interest to law enforcement. Some types of information, say the DNA on a rape kit, would lose interest very gradually, perhaps losing interest entirely when the rapist is almost certainly dead of old age. Other types, perhaps the appointment calendar, would lost interest much quicker. Assuming law enforcement knew who the person was likely to see, the calendar might be of no interest once the day of the appointment is past.
It's now been more than 3 months since the San Bernadino shootings, so my guess is that much of the information has decayed into relative meaninglessness. We don't know the information, so we can't tell for certain, but I think it likely The longer it takes the FBI to access the data, the greater the chance it won't be helpful.
My point: fast access to the data is worth a lot, which should be a consideration in determining whether and under what rules law enforcement gets access.