is there a time limit to return a trade-in

If using the online trade-in, you will be sent a shipping kit for each device. Devices are returned, assessed and a credit value assigned. The trade-in value is then returned to you by way of an Apple gift card for each device - and these can be used to purchase whatever you wish with Apple.  Apple generally expect an online trade-in to be returned to their trade-in partner within 14-days of you receiving the shipping kit.

If a trade-in is initiated during an online device purchase, you will usually pay full price for the item - but will receive a refund against your credit card upon subsequent return (and acceptance) of the item that was returned for trade-in. The shipping kit is usually sent to you within a day-or-two of your online purchase being shipped - allowing you adequate time to transfer data from old to new device before returning the trade-in.

Many choose to complete their trade-in in-store - as doing so removes any uncertainty as to the trade-in value or acceptance. The agreed value is credited immediately against an in-store purchase.

If you intend to buy now - and return the trade-in later - you can do so. A stand-alone trade-in can be initiated at any time - but the accepted trade-in value will be returned to you by way of gift cards to be used against subsequent purchases at Apple; the gift cards cannot be used retrospectively against a prior purchase.