This depends on the amount of coolant in the exhaust system. If it has really built up, it can take quite a while. Give it a good long run with plenty of WOT runs. The more WOT, the better. It needs the increased exhaust flow and pressure to heat the exhaust system up and push the coolant out.
If it's not cleared up within a couple of hundred miles, with plenty of WOT, something may be wrong.
One car I worked on had so much coolant in the exhaust system that I could shake the mufflers and hear the coolant sloshing around.
Remember vapors can look very similar to coolant smoke. Coolant smoke will stay low and not disappear very easily, vapor will rise quickly and disappear fast.[/
Would it hurt to drill a small hole in the muffler?, this used to be a common practice with a new muffler.