There's more than one significant variable here, and I think it perilous to generalize any one persons experience. But perhaps reading about various peoples experience at this can help give a sense for the different ways such a separation can go.

As mentioned before, age and "life situation" is certainly significant, whether children are involved, the level and type of commitment between the temporarily split couple. And just how solid the relationship is, and how comfortable the "left behind" person is in managing the various day-to-day aspects of life. For this latter issue, I figure the upside is that should I die first, my wife has had a good deal of practice at soldiering on without me! :-)

I have a friend who for some years had literally never spent a night apart from his wife, and the prospect seemed daunting to them both. In contrast, I've done three thru-hikes now, for the most part without my wife involved, though she did train and join me for stretches on two of them, and visited me mid-way on the third. These "reconnect" times were certainly helpful. But we've been married for over 30 years, and our "relationship" is pretty darned stable --- kids are grown, we still like each other quite well (!), but can make do on our own for extended periods. Just makes us appreciate what we have when we're back together.

All that said, I speculate that I won't do any more multi-month solo trips, but rather do a combination of shorter (up to a month or so perhaps) trips and trips that she and I do together.

I've known various folks that leave a S.O. at home while they thru-hike. Relationship stresses is certainly one thing that can derail a thru-hike, so I guess in general I mostly know about success stories. It's certainly more about the attitude and expectations of the person left at home, though certainly some long distance hikers find themselves more homesick than anticipated.