It doesn't matter, looking at my harvesters they have an intended deployment time of 6 months, the first 2 are upto 56 months. I might send them for some recreation soon :s.
Morale is down to 25%.
Looking a bit deeper, it seems after 6 months morale drops from 100%, it's down to 50% by 12 months, and 25% by 18 months, then perpetually stuck at 25%.
There's one other thing I've noticed, the morale is only properly visible from the task group window, on the individual unit details window every ship with less than a year deployed shows 100% morale, every one with more than a year is showing 0%.
And as evidence that morale isn't affecting harvesting rate, every ship in the Barnards star swarm has 7% fuel load, surely those with double morale would be harvesting faster if it made a difference.
But to be honest I usually put long deployment times on my harvesters and send them for R&R every few years.
I just didn't bother here because I was in such a pinch :p