It is when it interferes with stuff you actually care about. Like when I'm supposed be having family dinner or playing with my daughter and I realize instead I've just spent the past several minutes staring into the distance thinking about a work problem.
Sometimes I really envy people who never get the urge to work (either actually or mentally) on work after leaving the office at 5 pm.
I've learned to accept that work should only be a certain percentage of your life. Occasionally it can bleed over, sure, but unless you want it to be 100% then you need to drop it and think about other things. Maybe I'm just lazy, but when I'm with other things I care about - especially people - then work can kiss my ass :)
You don't have to convince me...it's my brain you need to talk to :)
I've gotten very good at sticking to a 40 hour work week and haven't worked a weekend in over a year. I manage to leave the office before 6 pm at least 9 days out of 10. The 'problem' is that once someone dumps an interesting math or programming problem in my brain then it takes over everything and I can't let go of it.
Sometimes I really envy people who never get the urge to work (either actually or mentally) on work after leaving the office at 5 pm.