We already now that VOY despite being more advanced than the Ent-D is in fact slower, after all in "Where No One Has Gone Before" it'll take the Enterprise over 300 hundred years to traverse the 2.7 million ly. So the maximum number of years would be 399 otherwise it would be over 400 years.
But then again it's always plot speed.
They picked the figure 70 year figure for a number of reasons
1.>To make the journey sound long impling that they wouldn't expect to see home in their lifetime.
2.>To make it easy for the audiance to follow, because obvioulsy the audiance could follow the fact that one season might span a few years.