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Hi, I found that an exposure of around 2 - 3 seconds with ISO 200 (using a digital SLR) was good for the first twenty or so minutes after the comet becomes visible to the naked eye. As the sky gets darker, I switched to ISO 400 and anything from 5 to 15 seconds, with 10 - 15 seconds being optimal. Bear in mind that I was using a 70mm f/4 lens on a fixed tripod. Anything over 10-15 seconds shows too much trailing for my liking.
As the others have said, although you use more film, it's best to bracket and take as many shots as possible.
Also, consider getting your photos scanned and burnt onto a CD when you get them processed. A little tweaking in Photoshop (or similar) can make a huge difference.
Hope this helps.
Morton
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