There are far too many variables involved to accurately predict how long each of your batteries will last, as you will use them. On an individual battery basis, part of it is just a random quality control in the making.
Your question seems to come from the point of view that an old camera will not be usable once the rechargeable batteries are exhausted. You probably have very little to worry about this point. Batteries are still available for many DSLR cameras that have not been sold new for more than 10 years. For example, one can still buy BP-511 batteries from quality third-party manufacturers for an EOS Digital Rebel (the first original sold as 300D worldwide) introduced in 2003. The latest Canon DSLR to use BP -511A (a slightly superior version of capacity, fully compatible with previous versions in both directions), the EOS 50D was discontinued in mid-2010. The case is very similar for the Nikon EN-EL9 battery used to power the Nikon D50 and several other cameras. The D50 was launched in June 2005. The latest Nikon DSLR cameras to use the EN-EL9 battery family also appear to have been phased out by 2010.
I have Lithium-ion LP-E6 (OEM and third party) batteries that I have owned for seven years and are still perfectly usable for my Canon DSLRs. The older ones do not last as long with one charge, but there is not a huge difference. Maybe a reduced capacity of 15% compared to my most recent ones?
Overall, these batteries have powered the last four camera bodies for more than 250,000 frames and, with the exception of Canon's oldest OEM battery, they still work very well.
The only battery I have ever had "dies" to the point of no longer being able to be used is the Canon OEM battery included with a camera purchased in 2011. None of the third-party batteries I still have, including a also purchased in 2011, have degraded anywhere near that much.
Of the four Canon brand LP-E6 batteries I've owned:
- One of them was thrown into a hole and lost in the wall of a building only 2-3 years after its construction in 2012. It worked well at that time. As far as I know, he is still inside the wall.
- One, made in November 2011, is pretty much "dead" and has retired. It will still take a charge, but will only last a handful of shots in any of the cameras. The "Charging Performance" display in the camera menu shows a single red bar for this battery.
- The other two are still fully functional with a slightly reduced capacity compared to their start date. The manufacturing dates listed are January 2015 and February 2014, respectively. They only show two out of three green bars on the "Charging Performance" view from my Canon cameras.
Of the ten LP-E6 third-party batteries I bought between 2011 and 2015:
One of the two vehicles purchased in April 2011 was lost when a velcro flap on a camera bag was loose and opened during the course of the week. a night photo shoot in a large natural area. I finally found two of the three batteries that escaped. This battery worked well when it was lost about 2-3 years after the purchase.
Two of them are packed with a Canon 7D whose use is rarely used, but they work properly when they are loaded and used on occasion. One was purchased in April 2011, the other in April 2012.
- The others still work well enough to stay in regular rotation in my regularly used cameras. The oldest is a third brand "Maximal Power" bought via amazon in 2012, the most recent are two batteries LP-E6 "STK (Sterling Tek)" bought in 2016. They display all three complete green bars in the "performance of recharge »" display.¹
Of the 11 LP-E6 batteries currently in rotation:
- Canon OEM (manufactured in 2014 and 2015) and five of the third-party batteries (Two "Maximal Power" purchased in 2014, two "STK" purchased in 2016 and the "Watson" included in a promotional package with a camera purchased in 2015) have the current firmware and are functionally identical in all my Canon EOS cameras that use LP-E6 batteries.
- The three older "Maximal Power" third batteries, all bought at one time or another in 2012, will power all my active cameras, but the 5D Mark III and 7D Mark II will refuse to display the serial numbers, the exact percentage remaining, or recharge performance for these batteries. These things first showed with the 5D mark III until the firmware update of the 5D Mark III in 2014.
As a result, the two Canon OEM and third-party batteries purchased in 2014 or later are the most used in the two devices I use the most, the 5D Mark III and the 7D Mark II. The other three third-party batteries are used in the 5D Mark II that I use when shooting with a three-body configuration or, occasionally, with a two-body FF configuration when shooting groups or of events in a very dark place.
I normally use two batteries with the 5D Mark III and the 7D Mark II and I can continue all day to record more than 2,000 images in one or the other without it. it is necessary to change the game if they were fully loaded at startup. At the end of a long day, they usually have 40% or more left. As the third body with the widest angle lens mounted on it, I normally have no grip of the 5D Mark II and rarely shoot more than a few tens, or even at most, a hundred views with this one. I usually exchange the battery inside every two or three times when the charge is reduced to around 60%, but most of this loss usually comes from long time intervals between the time it is loaded / used.
I also have several Li-ion batteries BP-511A, bought between 2009 and 2012, for a Canon EOS 50D. They always charge and operate the camera. But it's been years since I've shot enough with the camera to get an idea of their duration. Models 50D and BP-511A do not have a menu item to display battery recharge performance.
¹ As I have never seen a third LP-E6 display anything other than the three full green bars, I suppose it's possible that third-party batteries will show a full recharge performance value even if their performance is not higher than that of the monitor. Canon OEM batteries with reduced function. But my experience has been that quality Third-party batteries such as STK and MaximalPower will last longer charge / discharge cycles before starting to display a slightly reduced capacity compared to Canon-branded batteries.