If your good at pharmacokinetics, please help with this question?

Regarding Vancomycin, troughs are usually drawn before the 4th or 5th dose so that patient will be at steady-state. What about if a patient has SCr of 4.2 and is initiated on Vancomycin q48h or q72h? I don't think we would wait a week before assessing therapy. Shouldn't a trough be drawn earlier? When? (Assume no acute renal failure). Thanks for your input.