How long does a 1 TB backup take?

The answer depends mainly on sustained throughput. For 1 TB of decimal data, these are useful theoretical baselines.

Why can a backup take much longer?

Backup software may scan files, calculate hashes, compress or encrypt data before sending it. Millions of small files can also behave very differently from one large sequential file. Source and destination storage speeds matter as much as the network once the link is fast enough.

Planning a backup window

If a backup must finish overnight, calculate the minimum required bandwidth first and leave headroom for overhead and other network traffic.