While I don't have the right kind of patience or talent for 3d modeling (I discovered that long ago) I know exactly what you mean. I'm a coder, and I know it's time to stop when I start thinking about "real life" issues in programming terms. Although it is kind of fun.
Also when I erase something off a whiteboard by mistake, absentmindedly break something, etc, my first reflexive thought tends to be "Undo! Ctrl Z"!
When you are looking at every single article and tries to complain that people didn't indent their paragraphs, or every single word that remotely assemble for, if, while, i, temp, status, and int, you know you spend way too much time writing codes.