Think of your retro not as a meeting, but as a process. That process has two parts - asynchronous and synchronous. How much you do asynchronously is totally up to you and how adept you feel with working this way. That said, it’s best to give people async time to provide inputs and for you to synthesise those inputs meaningfully. So I recommend a week for this part of the process. The rest of the process can easily happen within a 60 minute window. If you haven’t had retros for a while, make it longer. 

Use the retrospectives templates on this site, to guide you through the process. For more information, read the detailed article.