SQL Server 2016 in-place upgrade time for large databases
I'd really recommend not doing an in-place upgrade; certainly not for a very large system like yours.  The reasons for this recommendation are many, and have been covered extensively by Mike Walsh in his answer on a separate question.

If you really need to do an in-place upgrade, the only real way to determine the amount of time required is to do a dry run on separate, identical, hardware.  This would involve installing SQL Server, and using backup-and-restore to copy all 1,000+ databases to the test server, then performing an in-place upgrade.  I'd recommend doing this in the very minimum since it allows you to test SQL Server in your environment prior to committing to it in production.