This could very well be an accurate estimate for some families. However, the blending depends on so many, many things. Goodness, discussion on this topic could be endless.
Think about the questions that would need to be answered in order to get a good picture of the possibilities for families to blend...
How many children involved, their ages, sex and are they all living under the same roof all the time, or are a few of them week-end & Holiday visitors?
The key, I'm sure, as to how long it takes to truly "blend" and how rough or smooth the road may be, is how well the parents put their plan together. Did they have lengthy, serious discussions on this, before getting married? Did everyone seem to get along well, during the relationship, before becoming a family?
A really BIG factor....Do the exes seem cooperative and want only what is best for the children? Do the "new" spouses and the exes have a civil and respectful rapport?
Seven years seems like an awfully long time for this to become a reality. Some families work at it harder and more consistently than others.....so it may happen quite quickly and easily for some. The fact is, some people never seem to be able to blend, which is sad for all involved.
So...seven years could be an accurate figure, that might be the result of a survey or study, but I'm convinced that with more effort and a lot of love, understanding and concessions from all parties, that average can easily be reduced to 4 years, maximum! I'm an optimist.