I only do time-based rates for vague work products, like consulting. Usually it's a quarter-day/half-day/day/week/chunk rate depending on the arrangement, and I

I only do time-based rates for vague work products, like consulting. Usually it's a quarter-day/half-day/day/week/chunk rate depending on the arrangement, and I try to limit my exposure to time-based rates, especially hourly, because they often give clients permission to eat up all my time whether or not I want them to. For more concrete work products (with deliverables), I do fixed rate billing.