Kodak E100VS works quite well with little to no additional exposure out to one minute. Beyond that, you might want to add 1/3 stop more exposure to 3 minutes, and 2/3 stop more exposure beyond 7 minutes. There is sometimes a slight blue shift in very long exposures (beyond 4 minutes).
If you are doing this in the daytime, and using ND filters, then you have another factor to consider. Some ND filters are not perfectly neutral. On longer exposures, any colour cast from an ND filter will alter the final result on film. You might not see, or notice, a colour cast on shorter exposures, but as time increases the effect can be more noticeable. Buy good ND filters.
Ciao!
Gordon Moat
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