How long will 4x 0.16A Computer Fans running off a 15000mAh Battery Last?
Total draw: 4 * 0.16 = 0.64A

Total capacity: 15Ah

Approximate time: 15/0.64 = 23-24hrs 

That assumes the labels/ratings on the fans and power pack are accurate.  Most cheap battery packs have much less then the rated capacity, and it falls off with time and cycles too.