This is a 72hr heat pack. The 96hr ones are out of stock everywhere! However the 72hr ones are working great, I haven’t had any issues.
If the daytime temperature in the next week after your purchase is forecast to be below freezing, you should add a heat pack to your order. However, a heat pack can damage your plants if the outside temperature is too high. I usually check the temperatures where heat packs are going and I might remove the heat pack if I think it will be too warm. In this case, I will not refund the heat pack.