I did mine at about 56K miles, or nearly 4 years, just after the car went out of "free" maintenance. I was having a small leak, and was hoping that it would clear up during this.
It didn't, at least until BMW did a recall fix for the EGR cooler on my diesel model in late 2019.
Don't trust the "lifetime" claim. Which I feel is mostly specified so BMW isn't obligated to do it during the maintenance. Which used to be 4 year/50K miles. And is extendable. If they recommended coolant change at 40K miles, the people would expect it to be "free"
I'm considering getting a quote for transmission fluid drain/fill and transfer case at some point, since I feel that this would be cheaper than getting a new car in the next few years.