Noise reduction in bulb setting.

I'm aiming to take some long exposures using a remote timer on a 5D but I've just started to wonder what the noise reduction function will do?

Normally it compares to a dark frame exposure of the same duration but will the camera know the exposure duration to make such a correction against?

Anyone know the answer to this one?

Good question. Never thought of it this way, as I never use the NR feature. I like the freedom of being able to see the picture right after the shutter closes. After all, the noise isn't that bad without NR IMHO.
It's possible that the camera has an internal clock that records the time elapsed while the shutter stayed open, and will apply that for the NR exposure.

i know that my 10D can tell me the exact number of seconds that I hold my shutter open for, it shows up when i review my images

ISTR posts that reflect it functioning normally, i.e. running the NR sensing for the same time as the original exposure.