Upgrading to SQL Server 2017 from SQL Server 2012 time allowed We have a customer running a SQL 2012 database for their document management system but this needs to be upgraded, They want to upgrade to SQL Server 2017 and looking at the forums the best method is side by side.
Can I ask what sort of time should I allow for this type of upgrade - I need to inform the customer of the downtime.. Approximate size of the database is 800GB.

Any help or advise is greatly appreciated.
Backup restore depends on a lot of different factors, the biggest ones being


Your network
Your storage


The best way to ever give a good estimation is to test it.

Setup the second server/dev env, and do the migration as a test, this will give you a very good estimation of the time it will take.
And as a second huge benefit, you'll have all scripts ready + experience for the production migration.