I know its time to take a break when i get home from work and hate the thought of going to the gym. The second the gym stops giving me pleasure means that i am over worked. I feel that this is a sign that i am overworked!
This doesn't happen at regular intervals. But, if i had to say when, i guess evern 3 months or so.
Spring, summer and fall I take a week off every two months.
In the winter I take a week off once a month.
My job has me on 24 hour call one week a month all winter, so I take that week off.
I need more time off than most. I lift heavy all year, and as a result, I will burn out fast if I don't take a good deal of time off.
I get sick a lot for some reason, usually about 3 or 4 times a year. At those times I usually take my time off. Not ideal conditions, but I'm out of the gym anyway, so...