I did a show for two and a half years. I came in two and a half years into the run, and after I left it ended up running almost 8 years, total. After a little while, it really became almost like a day job: show up, do the show, go home, nothing much changes. But we always had different actors in and out, so that's sort of how it was kept fresh.
And trust me, I HAD challenging cast members. "Jane*, here's your notes from last night. Could you look over that scene with your daughter at the top of Act II? A couple of lines got mixed up in there."
"I'VE BEEN DOING THIS SHOW FOR FOUR YEARS, I DON'T NEED TO LOOK OVER ANYTHING; I KNOW MY LINES YOU LITTLE TWIT!"
Yeah.
With the playwright still making changes, it sounds like you've got no trouble keeping it fresh, but keeping it sane. I was in much the same boat, as well, as the director/playwright was at most of the shows, and constantly changing things. When other people's insantiy (truly insane, not just obnoxious) started impacting both my job and my health, I put up with it for a few months, and then realized that I didn't have to, and left. It happens in long runs, in both the technical staff, and the actors.
If you get burnt out, it really is okay to leave a long running show. It's not beating you; you've got a specific tolerance for this work, and you've reached it. Acknowledging your limits is better than forcing yourself to work past them until you crack and something rather bad happens.
I've got a theory that theatre folks just aren't wired for long hauls of repetition, which is why we're so mercurial about employment. Six months here, 10 weeks there. Think about it; that sort of constant uncertainty would drive most people into an institution, but we take it as normal.
*Names changed to protect the guilty