How long can a firewire cable go before I will start to see slowdown in the transfer of information?

I want to run some firewire that is about 100 feet long (if they even make it this long)...

I don't see any 100' firewire cables on B&H, but I did find this: http://www.bhphotovideo.com/c/product/472600-REG/Datavideo__FireWire_6_pin_to_6_pin.html.
You could also provide your own cables and buy the firewire repeaters individually: http://www.bhphotovideo.com/c/product/441528-REG/Gefen_FW_141_FireWire_Repeater_400.html.

I don't know the limitations of firewire, but the fact that B&H is providing a kit with three 66' cables and two repeaters seems to suggest that 66' is about as far as you can go reliably without needing a repeater.

Also, I may be wrong about this, but I think that an over-lengthy stretch will not slow down. What it will do is give you errors or just plain not work. Again, not 100% sure on that last bit of info.

EDIT: found this on good 'ol Wiki:

Cable length is limited to 4.5 meters (about 15 ft), although up to 16 cables can be daisy chained using active repeaters, external hubs, or internal hubs often present in FireWire equipment. The S400 standard limits any configuration's maximum cable length to 72 meters.