Hi John,
Downtime for the system copy depends on the following parameters
1) Number of CPUs available on the source and target system
2) Number of application servers available on the source and target. IT will be useful in using distribution monitor.
3) What is the approach used for export- > Table splitting, Package splitting.
As a best practice you need to have trial run done for getting exact estimates.
Hope this helps.
Regards,
Deepak Kori