Measuring with a flexible tape measure, I come up with a hair under 30" with at least 1" travel either way on the adjuster slot. Dayco lists #5060295 a 29.53" 6-rib belt which fits '17-'22 BMW M760i plus some older Infiniti and VW models.
The part number is good though as my local NAPA lists generic ones for around $32. Little pricey for a belt but it will last forever.
The one I was using previously with the alternator in the OE position was 39.33" long. I'd say a 10" shorter belt is a "gain".