Upgrading to SQL Server 2017 from SQL Server 2012 time allowed
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.