Thank you, everyone, for all of your wonderful suggestions. They've really helped me think about how to talk about it differently.
There are a couple of things that do kind of stink about the situation and make it harder to talk about. For one, DH is entering year 7 of his PhD (and he started the darn thing with an MA in hand, no less), which is much longer than anyone (him and his advisor included) were expected at the outset. So the original plan, when he was young and full of hope, was to graduate this past spring. Which, of course, totally didn't happen. He's a little frustrated with that, because he's more than ready to be done. I am totally fine with the timing, except for wishing my DH wasn't unhappy about it himself.
The other thing is that, since DH feels like he just wasted two months, he really doesn't feel like he's able to take a vacation from writing when we visit Home City. I've encouraged him to, mostly because he needs real breaks from work from time to time, and vacation is always a good time to do that. But if his writing is going well, he often won't stop while it is so that he can take advantage of the momentum. If I can convince him to take a break, or at least to work when it's not super obvious he's working, I probably will use the "Please don't talk about it because we both need a vacation from it!" tactic, though.
Has anyone in your/DH's family ever written a dissertation? Because they may be underestimating what a huge effort it is. Trust me, I sympathize. I make my living writing, and you wouldn't believe the number of people who think that you sit down in front of the computer and POOF the words just appear.
In my family, nobody has a PhD. My dad has an MA, but it's more of a professional degree that didn't involve writing a thesis. So for them, I'm positive that they just don't really get
what it's like to write a book. I certainly didn't until I watched DH try to do it, and even I don't really know what it's like in the end.
In my DH's family, the nagging stings a bit more because it mostly comes from his mother, who has a PhD and has been an academic for her whole career. I swear she's Superwoman, though, because she finished hers in like 4 years (with three children and a husband in tow). Aside from that, her PhD is in a field that seems to be different enough from DH's that she doesn't actually seem to have a good understanding of how much research his dissertation entails, so she's ended up with some sort of unrealistic expectations about the amount of work necessary to finish.
Perhaps the family need also be reminded that done isn't good enough. It also has to be good enough to be done.
Is your DH in a field where his dissertation may later be published? Perhaps use that to spin the time commitment and make them understand. i.e. "It can be frustrating, but DH is working so hard to finish with a strong dissertation. Did you know that with a great dissertation he'll 1) be so much closer to publishing his first book or 2) be a much stronger candidate in this tough economy. Think of it like building a house. Do you want the builders to finish fast or build it strong?"
That's a good point. DH is going to be interviewing for jobs on the strength of his dissertation (among other things), and my parents know the job market for academics is pretty pitiful right about now, so that might be a good way to answer any questions they have about the timing without getting too confrontational.