Although this does not answer the regulatory question, there's an interesting Technical Bulletin about Duracell alkaline batteries at
http://www.duracell.com/media/en-US/...20Bulletin.pdf
Among other interesting facts, the Bulletin indicates that at 70F the batteries lose only 3.5% of their capacity per year. They retain 85% after four years and better than 70% after eight years. Shelf life is worse at higher temperatures and better at lower temperatures.
To directly address the question of battery replacement for the ubiquitous yellow ACK E-01 that uses standard drug-store-variety Duracell MN-1300 D-cell alkaline batteries, the manufacturer's guidance appears at
http://www.ackavionics.com/pdf/E-01%...9.04.20071.pdf and staties in pertinent part:
"The battery replacement date is found on each Duracell MN1300 cell. It reads as follows: 'Best if installed by (date)'. The (date) indicated is the date by which the batteries must be replaced. All cells must have the same date."
Other than the ACK E-01, every ELT I've ever seen use special battery packs that are clearly placarded with a replacement date. I personally have never encountered a situation where the replacement date needs to be calculated on the basis of the 50% rule of 91.207(c)(2). My theory is that the FAA lawyer who wrote that regulation inadvertently omitted the word "OR" from the sentence, and that it should actually read:
"(2) When 50 percent of their useful life (or, for rechargeable batteries, 50 percent of their useful life of charge) has expired, OR as established by the transmitter manufacturer under its approval."
But what do I know?