How much should I inflate my tires? I just purchased a new bicycle pump with a pressure gauge and after checking my tyres, inflated them to the top end of interval printed on the tyres (4.5-6.5 bar). 

I'm wondering when the tyres should be in the lower range of the interval, and when the higher end is appropriate. Is it a question of weight? Type of terrain I'm going to ride?

The tyres in question are Schwalbe Kojaks, mounted on a light urban hybrid bike.
My general rule of thumb is 10-15% lower than the posted maximum for normal road riding. Higher if you're carrying a load or are a larger rider, lower if you're a lighter rider. There's an article in Bicycle Quarterly that's a pretty good guide on tire pressure for road riding depending on weight and width of tire.

http://www.vintagebicyclepress.com/images/TireDrop.pdf