Always worth doing DIY things like this to save you the BMW service rates. Varying opinions as to how regular you should change but 3 years seems to be the average from a quick Internet search. At 7 years would certainly do it as it can degrade in terms of its anti freeze and protective capability. Full flush through prior to a refill with the appropriate quality and mix ratio. I go for a change at 3 years which puts it in betewwen an oil change at 3 years and with an oil change at 6 years. Good for resale value as part of the service record as well.