It depends on the frequency of play, I think. I tend to play strategy games only about 6 hours per week at most, sometimes as low as 3 hours. But I play them for a very long time in months. In terms of AI War, it's never gotten old on this schedule, largely because the game keeps changing and so forth. I play 4-player co-op, and the dynamics of us cooperating (or failing to do so) also tend to make the game a lot more interesting over the long haul.
If you just play solo, and play a huge number of hours per week... I think that with any game, past a certain point, you're going to burn yourself out, plain and simple. Partly for the obvious reasons, but also because you don't have time to mentally recharge.
When I was in high school, I played tennis 2-3 hours per day, 6-7 days per week, for three years. It was intense, and I was in extremely good shape at all times, but my actual skills at the game would tend to plateau despite whatever smaller improvements I was incrementally making to my serve, my net game, or whatever.
However, on occasion I would have a break of a week or two, either because it was between seasons or it was family vacation, or a holiday, etc. During these times off, tennis would fade out of my life for a bit during a welcome break, and it would stop filling my subconscious and my dreams. And yet apparently my subconscious would still be working away, because when I'd come back after the break, I'd be jittery and rusty for the first hour or so, but then markedly better after that -- for good, usually, unless I fell back into some bad habits like lazy footwork in 100 degree heat.
My point is that when you're engaged too intensely in any single activity for too long a time, your ability to innovate and process goes way down. You start falling into ruts, and the whole thing can be frustrating. I can remember giving up in tiebreakers in some second sets in ladder matches because I simply wanted to go home rather than being forced into a contentious third set. That third set would have been winnable, but it would have been just way too much of a grind and I was tired, bored, etc. I knew just how it would play out, despite never having played this opponent before.
So what I'm saying is that I know how those folks feel, but I don't think it has anything to do with PvE, or any specific game. I played Counter-strike ridiculously frequently for a few years -- and GoldenEye before that, and Mario Kart 64 around that same time -- and those all fell into the same ruts for me. I've learned that the key to my personally being able to enjoy games for a long time is to take them in some moderation, rather than all in one big guzzling gulp. Sure, when there's a new game or a new expansion or a set of new features that are exciting, it's great to play long and hard for a while. I still do that. But that sort of intensity just can't be sustained indefinitely, for months and months, is what I mean. Eventually everyone burns out when they try to do that with any game. Hell, that even happened to me a bit when I was in Chess club, because I was just playing way too much Chess. After I was out of Chess club and playing online on Yahoo! more occasionally, my online rating shot up 200 points. Go figure.
