Is this measured chronologically? (In years, for example) or in cycles?

Both, I think. At least they're both factors, but I don't think it's possible to reliably predict the useful lifetime. Batteries age whether you use them or just let them sit on the shelf, but they age faster if you put them through a lot of charge/discharge cycles. I'm no expert, but I suspect other factors like temperature and charge during storage have an impact on a lithium ion battery's lifetime.

All these variables probably combine to create a range of possible lifetimes that's probably too large for battery manufacturers to provide a useful estimate. If they guess too low (in either time or cycles), people will accuse them of giving a short lifetime to encourage unnecessarily frequent battery replacement. If they guess too high, people will complain when their batteries don't meet the estimate.

The best thing to do, of course, is to monitor the battery's performance. Does it recharge to the same voltage? Does it provide useful capacity? It can be hard to really know when the performance has dropped if you're not paying close attention. To that end, some cameras (e.g. Canon 6D and recent 5D variants) keep track of each battery's performance so that you can see when a battery might need to be replaced.