Basically reciprocity failure is when the film stops responding to increased exposures in the normal way resulting in the wrong exposure so you end up under exposing so you have to compensate by giving a longer exposure than you think. A very rough rule of thumb is to double the exposure time for exposures much over 1 second but the effect gets more pronounced the more over one second you go (ie not constant) and also varies from film to film. Bracket fairly widely is the answer and for really long exposures you may well get a colour cast as well. I have not looked but you may well be able to find more detail on what sort of correction you might need for your Velvia on line, there used to be tables of time versus estimated correction for each film.
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