You have 14 days after you receive your new devices to sent the trade. See the following from Apple Trade In - Apple
How quickly should I return my device?
Your trade-in estimate is valid for 14 days after you receive your new device. It’s important that we receive your trade-in within this time frame to ensure that you get the estimated value.
Regarding the credit, see the following from the same source as above:
If I buy a new device online with a trade-in, when will the credit be applied?
It depends on what you buy and how you pay for it.
If you buy an iPhone, iPad, Mac, or Apple Watch and pay in full, your credit will be applied to your original payment method up to the total purchase price after we receive your trade-in device and confirm that the condition matches what you described. For any remaining amount, you’ll receive an Apple Gift Card by email.
If you buy an iPhone, iPad, Mac, or Apple Watch with a trade-in and choose a monthly payment option, your estimated trade-in credit will be applied directly to the purchase price of your new Apple device. Once we receive your trade-in device and confirm that the condition matches what you described, there’s nothing more you need to do. If it doesn’t match, your original payment method will be charged the difference in value. In that case, we’ll work with you to confirm that you still want to move forward with the trade-in.
