I find it's usually 565mm butsome can be 5-10 mm different either way, which can make the pipework look a bag of bollx if your coming horizontally out of a stud wall! I usually get the customer to get the rad in when I start first fix or tell them it may look crap if not!
Normally 550-565 depending on the make, I normally make sure I know exactly what is being fitted before I first fix so I can look up the technical data on it.