BMW does NOT have a service interval for the Coolant, it is a "lifetime" item. I think any body other than dealer would say 5 years or 150k miles. See this article
http://www.aa1car.com/library/bmw_co...ice_system.htm
So, in a nutshell, unless there is a mechanical problem with your car that requires replacement of the coolant, it will come out of your pocket.