This is too broad a concept to have a universal answer. The commercial world, independent film world, and the Hollywood worlds all have radically different answers to this question, and even then there are always exceptions.
If this is an editing gig for someone else and it isn't a favor for a friend, then I'd go for an hourly or at least a day-rate and NOT EVER a flat rate for editing the project. The worst thing you can do is agree to a flat rate to edit the project because then you are in for revision hell because they know they have you for the same price until it's done to THEIR satisfaction...