When you work a full-time job and essentially write full time (or very close to it), you have to consider how you manage your schedule. I've decided to do these massive pushes, where during a 2-4 week period, I do enough work that I can take a little time off. And by time off, I mean that I only write seven to fourteen hours during a week, and I can spend the rest of the time decompressing on my own projects. Meaning playing games. Pursuing hobbies. Trying to get back into the headspace for the next big push.
The alternative is to keep a constant 60-70 hours of work a week, without any breaks, and I can't do that. I'd burn out and go a little crazy. The work would almost certainly suffer.
When I was a game designer, I had a 100,000 word month (That didn't include edits, I had to produce 100,000 new words). It followed another big month of writing and at the end of it, I couldn't function as a writer any longer. My notes coming back from the editor amounted to "Are you okay? Do we need to send help? Do I need to hire Liam Neeson?" I learned at that point that I had limits. I can work hard in bursts, but drive too hard over too long a period, and I burn out entirely.
I don't have any great advice for learning your limits. For most people, the only way to do it is to exceed them and crash and burn. The best advice I can give you is that when you know it might be coming, have someone watching your back, ready to tell you when your text turns to pudding.
Otherwise, your editors might come to the conclusion that you've been kidnapped and your terrible writing is all an elaborate code. Either that or that you've just lost it and are no good to them any longer. But I'd shoot for the first one, you might get to meet Liam Neeson.