Mine went @ about 45,000 miles - there was no apparent drop off in performance, but my newly purchased Durametric logged the fault, and there was no CEL either. Porsche picked up the tab as I had an extended warranty.
There was a definate increase in mid range power with the new MAF.
That Durametric is a fine piece of kit to have in the garage - it paid for itself in one easy diagnostic as I wouldn't have picked up the faulty MAF without it......