How long does 1 TB take at 1 Gbps?

Using decimal data units and the theoretical line rate, transferring 1 TB over a 1 Gbps connection takes 8,000 seconds.

What can make the transfer slower?

Real transfers can take longer because of protocol overhead, latency, congestion, storage performance, Wi-Fi conditions, encryption and server limits. The value above is the ideal line-rate estimate, useful as a baseline for planning.

How is it calculated?

Transfer time = data size in bytes × 8 ÷ connection speed in bits per second. Decimal units are used here: 1 GB = 1,000,000,000 bytes and 1 TB = 1,000 GB.