It depends on your target market (who is your audience), what country they are in, target demographic (older/younger, male/female, what state of mind are they in relation to your offer), product being sold and how the conversion funnel turns the user into a buyer (pure online conversion, phone-assisted?), how long does the sales funnel take on average to convert the user, etc.

With the answer to those questions we can sketch out the ideal time/day to send the drip campaign and then validate by testing