To be totally safe, you need to know the pack/kill date that is marked on the case the meat was shipped in. The cryovac needs to be puncture-free, no leaks at all. I double bag briskets in plastic food bags, like pan liners, to help protect the cryovac.
You also need to keep the meat in the 32-36 degree range for the entire aging period. You don't want it in a kitchen refrig that is always being opened. I use a small chest freezer that's controlled by something like this.
If I know the kill date I feel safe to age briskets up to 50 days from that date.
