The Clinical Journal of Sports Medicine uses the following format for Digital Object Identifiers:
Where xxx is a simple incrementing count, currently in the 200s, based on order of article publication. The latest issue has 26 articles, and they publish bimonthly, for a total of under 200 articles per year. Theoretically, if they increased publication to daily, they could reach 10000 articles per year. Hence, their 16 digit format will be sufficient for at least a trillion years.
Simple question: why? The usability is terrible; it's very difficult to transcribe by hand. They could chop the number of digits in half without any realistic risk of running out.