Nick here from Shopify.
Really good question. I looked into this for you and found some information on the Printful website itself. Printful state the following:
Any claims for misprinted/damaged/defective items must be submitted within 4 weeks after the product is received. For packages lost in transit, all claims must be submitted no more than 4 weeks after the estimated delivery date.
You can use this information to apply it to your store policies and can see it directly where it came from on their site here. Printful actually has a whole blog/ information section on Terms, Policies, and Returns which you can find here.
All the best, Nick