Here's this shows TL;DR:

• Why does every brand set a 30 day window?
• We see the majority of returns happen in the first 14 days
• Brands are likely just copying each other to always create 30 day policies
• Most returns happen early which means you can extend the length of your return window
• Three steps when looking to change your return window length
  • Benchmark against the competition in your industry
  • Make sure that you are aligned with the rest of your policy 
  • Evaluate your current conversion rate so you can evaluate how a change in your return window impacts it
• Your return policy is for your customer

Other helpful resources on this topic:

How to build a customer-centric return policy (ebook)
Setting a return window for the holidays
The different types of returns and how they impact LTV