For a rookie driver, I say closer to 6 than 5 hours. Let's see, and please chime in if I am off here:
La Romana to Santo Domingo--1:45
Crossing Santo Domingo--1:00 with luck, remember, rookie driver and all that construction, and depends on time of day.
Santo Domingo to Santiago--2:00 with luck
Crossing Santiago--there are 10 traffic lights and some horrible intersections for newbies-> 1/2 hour
Santiago to POP -- 1:20/1:40 safely without any big trucks in the way.
That adds up to 6+ hours with no pit stops, no "greenies" and no getting confused and lost.
HB