My six year old is on track to finish OPGR by the end of June. We started in Kindergarten, so it has taken us two years to work our way through. We don't do any review during daily lessons, but we did take a break last summer and worked through the first three ETC books. It really boosted his confidence to work through them quickly. When we restarted OPGR in the fall of first grade, the lessons were much easier and he made many leaps.
If I tried to do "2 Review and 1 New," I think I think there would be a rebellion.