The length of time that a package can stay in a parcel locker depends on the carrier and the post office if it's being delivered to a USPS parcel locker.
At my local post office they will allow the package to stay in the parcel locker for up to 5 days before removing it from the parcel locker and putting it back inside the post office.
Then they will wait an additional 15 days and during that time if they have your phone number they will attempt to call you.
If they can't get a hold of you and you still do not pick up the package they will send it back to the person or company who shipped the package.
Most people pick up their packages from the post office or parcel locker within a few days though.