How often do you take a break when on a long journey.

[JRvaux]

I am a working drummer and am going on tour this Friday for 10 days. I am driving my drums and myself all around the uk. The longest journey I have is from Aberdeen to Southampton which is 9 hours of continuous driving. I passed my IAM test back in 2002 so am probably a little out of touch. However what's the longest you would drive without a break? I can do up 4 hours before I feel that I must stop for a rest.