It's actually 90 days for cell phones, but the catch is, if you decide to cancel your contract after X days of the trial period, you will have to pay ETF . The 90 days only applies to the cell phone itself, not the contract.
Straight from their website:
If I do not like my phone, can I return it?
Costco's return policy for any Wireless Advocates merchandise is 90 days from date of purchase. This includes all wireless accessories and handsets. Within 90 days you may receive a refund on the handset itself once it is received back into our distribution center. However, you will still be responsible for any applicable early termination fees if service is canceled after the Trial Period. The Trial Period varies by carrier and state. See carrier Cancellation Policy for details."