In the fall/winter months I take one week off every 5 weeks, because my job is very stressful in the fall/winter, and I need to take a break that often just to stay healthy.
In the spring/summer months, I just go until I feel like I'm starting to burn out, and then I get out from under the weights for 1 full week. I'm taking a week off right now, as we speak.
I tend to get sick if I don't take breaks when my body wants them.
I also train heavy all year long, which I find leaves very little room for error when it comes to breaks/rest. Deadlifting 5/4/4/3/2 rep sets when my body is already on the brink of exhaustion will fuck me up in no time.