Mike Miller's Old School BMW Maintenance Schedule recommends power steering fluid changes every 30K, I believe. My indy mechanic whom I trust says it should probably be done after 15K, so I'll probably have it done whenever my car is in for something else between those two milestones. Agreed on losing power steering fluid from spewing out the top, hence the "shop rag and rubber band mod" that's popular on the track.
Good to know about changing the reservoir since it includes a filter, glad the reservoir is pretty cheap. And yeah, I agree it's very odd that BMW has neither a sensor nor a recommended service interval on this. Of course they no longer recommend changing coolant either.