The answer is, farther than you probably think.  While the IEEE1394 standard only specifies cables up to 15ft (4.5m) there are ways that you can extend that length.

I’ve succesfully run firewire up to 198ft using two in line repeater amps like this and three cables like this. You need to use high quality 6 pin firewire cables throughout because  Pin 5 & 6 carry power to the repeaters.  You also need to be plugged in to a device that can provide power on the line.

I ran this setup for years connecting a couple Canon XL1s cameras to a datavideo SE-800 mixer.  (Note that the mixer does not provide power on it’s DV inputs, so there was a firewire hub between the mixer and the first long cable.)