STOP DOING IT!!
If you read the manual - especially in slightly older car with power steering.... the power steering pump is taking strain 'cos it's still trying to "assist" even though the wheel isn't gonna turn any more.
I buggered up my mum's Espace by holding it at full lock too often for too long... and that was NOT a fun car to drive when the pump went around a rounadbout!
You can take it to full lock, but they recommend 3-5secs max and then jsut back off a shade.