Upgrade to SQL Server 2017 from SQL Server 2012 is allowed

We have a client running a SQL 2012 database for their document management system, but that needs to be upgraded. They want to upgrade to SQL Server 2017 and by consulting the forums, the best method is side by side .
Can I ask what type of time should I allow for this type of upgrade – I should inform the customer of the downtime. The approximate size of the database is 800 GB.

Any help or advice is greatly appreciated.