2-3 days depending on the brisket and the fridge. Is it cryovaced? If you have a good fridge that stays between 34 and 36 degrees you can keep a cryovac brisket for weeks in there. They taste better also. If you have a funky smell when you take it out of the packaging rinse the brisket and pat dry, the smell should be gone afterwards.
I bought a brisket last Tuesday and I'm cooking it next Thursday and its just sitting in my beer fridge.
Even if it's still a bit frozen it's ok to start cooking it. You may actually get a better smoke ring that way.