Time Frame for Actvating Upgraded Phone

Our family upgraded several phones during a current promotion, two of which are Christmas gifts. While ordering, it said they would not arrive until mid-December however the two phone which are gifts have already arrived before December 1st. How long can we hold the new phones before actually activating them? We are also dealing with trade-ins.

The only information I can find is that the old phone must be returned within 30 days of activation but can’t locate how long we have to activate the new ones.

Thanks!