How long does a 10 TB data migration to the cloud take? WAN latency and the TCP window trap

How this is calculated

The naive math (10 TB divided by 10 Gbps) gives 2 hours 13 minutes, and that is what most transfer calculators return. It is the number you'd see only if window scaling is negotiated and the link is otherwise idle. With the unscaled 64 KB window and 40 ms RTT the throughput ceiling is 64 KB / 0.04 s = 13.1 Mbps, which is 0.13% of the 10 Gbps link. Scaling opens the window up to 1 GiB, which pushes the ceiling past the link rate so the transfer runs at the full 10 Gbps. The planning question is which of those two regimes your stack actually lands in, and the answer depends on the OS, the transfer tool, and whether anything along the path strips the TCP options.

Verdict

A 10 TB cloud migration over a 10 Gbps private link at 40 ms round-trip time takes about 70 days 15 hours with the default unscaled 64 KB TCP window, or 2 hours 13 minutes once RFC 1323 window scaling is negotiated and the window can grow past the 12 MB the bandwidth-delay product demands. The 700x gap between those two regimes is the structural reason a naive size-divided-by-speed calculator is dangerous for migration planning: the 2 hours 13 minutes figure is correct only if window scaling is on, the link is idle, and nothing along the path strips the TCP options, whereas the 70-day figure is what you actually get when any of those conditions fails. The planning move is to validate the scaling regime with a small probe transfer before committing a cutover date, because the difference between 2 hours and 70 days is the difference between a routine migration and a missed deadline.