There really is no way of telling. It depends on the type of battery (NiMH, lithium ion, etc.), how many discharge/recharge cycles the battery has gone through, whether the device is plugged in most of the time, storage conditions,....

Sorry this isn't much of a help. But it's kind of like asking how many licks there are to a Tootsie Pop. :-)

In my limited experience these batteries are Li-ion and give approx 12 months of full performance and another 12-18 of decreasing performance before becoming basically useless. When I say useless I mean that the battery will no longer provide power for the length of a typical 1.5 - 2 hour movie.

Mine lasts about 3.5 hours, not quite long enough to watch 2 movies. Unfortunately, I don't remember the brand name, I bought it many years ago.

You can probably check the specs online with the manufacturer before you buy it. I also like to read product reviews on Amazon.com, even if I don't buy it there.