Theoretical max throughput of a FW400 connection is 50MB/s. Realistically, a drive connected via FW400 will get somewhere around 40MB/s.
If you're using FW networking rather than just target disk mode, there's more overhead, so I'm going to randomly guess in the vicinity of 25MB/s, though it could theoretically be faster. Meaning somewhat under a minute, basically--I'm going to guess 40 seconds.