Depends what you are driving...
If it's a car or car based van then as much as you want, there is no low that covers it (though if you cased an accident and it was shown you'd been driving for 30 hours non stop it wouldn't be looked on very well)
If you are in something with a tachograph then it's an average of 48hours a month and no more than 60, so aproximatly 24 hours in 2 weeks but this may be reduced (or increased slightly) depending on what hours had been worked in the previous weeks.