Shouldn't have asked!!
Acording to the Constitution, as amended, a vice-president who succeeds to the presidency on the death or disability of the president can serve up to half a term and still be eligible for election twice. If he succeeds one day before halfway through the term, he can only be elected in his own name once.
So if Romney was Newt's veep, and Newt kicked over two years and one day into his first term, or inded second term, Romeny could serve two years in succession and still be eligible to be elected President twice, which would total 10 years.
You would not believe how I happen to know that!
“Please explain how he would be president of 10 years...”
2 years of nomination campaign plus 8 years presidency, sorry to be unclear.