There are way too many variables involved to accurately predict how long each of your batteries will last for the way you use them. On an individual battery basis, some of it is also just random quality control in manufacturing.
Your question seems to be coming from a viewpoint about worrying that an older camera will no longer be usable once the rechargeable batteries have worn out. You've probably got very little to worry about on that score. Batteries are still available for many DSLR cameras that have not been sold new for over a decade. For instance, one can still buy BP-511 batteries from quality third party makers for an EOS Digital Rebel (The original one sold as the 300D in other parts of the world) introduced in 2003. The last Canon DSLR to use the BP-511A (a slightly higher capacity version that is completely backwards compatible in both directions) was the EOS 50D discontinued in mid-2010. The case is very similar for the Nikon EN-EL9 battery used to power the Nikon D50 and several subsequent cameras. The D50 was introduced in June 2005. The last Nikon DSLR cameras to use the EN-EL9 family of batteries appear to have been phased out around 2010 as well.
I've got Lithium-ion LP-E6 batteries (both OEM and third party) that I've owned for up to seven years that are still perfectly usable for my Canon DSLRs. The older ones don't last quite as long on a single charge, but there's not a huge difference. Maybe 15% reduced capacity compared to my newest ones?
In aggregate these batteries have powered my four most recent camera bodies for well over 250,000 frames and, with the exception of the oldest Canon OEM battery, are still going strong.
The only battery I've ever had "die" on me to the point it is not usable is the Canon OEM battery included with a camera purchased in 2011. None of the third party batteries I still have, including one also bought in 2011, have degraded anywhere near that much.
Of the four Canon branded LP-E6 batteries I've owned:
- One was dropped into a hole and lost in a wall of a building only 2-3 years or so after it was made in 2012. It was working fine at that time. As far as I know it is still inside the wall.
- One, made in November 2011, has pretty much "died" and has been retired. It will still take a charge but will only last a handful of shots in one of the cameras. The "recharge performance" display in the camera's menu shows a single red bar for this battery.
- The other two are still perfectly functional at slightly reduced capacity from when they were new. The manufacturing dates stamped on them are January 2015 and February 2014, respectively. They do show only two out of three green bars on the "recharge performance" display viewed from my Canon cameras.
Of the ten third party LP-E6 batteries I have bought between 2011 and 2015:
One of two purchased way back in April 2011 was lost when a velcro flap on a camera bag loosened and came open during a night shoot in a large natural area. I eventually found two of the three batteries that escaped. This battery was working fine when it was lost about 2-3 years after purchase.
Two are packed with a Canon 7D that rarely sees use, but they work fine when occasionally charged and used. One was bought in April 2011, the other in April 2012.
- The rest of them all still work well enough to stay in regular rotation in my regularly used cameras. The oldest is a third party 'Maximal Power' brand bought via amazon in 2012, the newest are two 'STK (Sterling Tek)' LP-E6 batteries bought in 2016. They all still show the full three green bars in the "recharge performance" display.¹
Of the 11 LP-E6 batteries currently in rotation:
- Both Canon OEM (made in 2014 and 2015) and five of the third party batteries (Two 'Maximal Power' bought in 2014, two 'STK' bought in 2016, and the 'Watson' included in a promotional bundle with a camera bought in 2015) have the current firmware and are functionally identical in any of my Canon EOS cameras that use LP-E6 batteries.
- The three older 'Maximal Power' third party batteries, all bought at one time or another in 2012, will power any of my active cameras, but the 5D Mark III and 7D Mark II will refuse to show serial numbers, exact remaining percentage, or recharge performance for these batteries. These things did initially show with the 5D mark III until a 5D Mark III firmware update in 2014.
As a result, the two Canon OEM and the third party batteries purchased in 2014 or later get used the most in the two cameras I use the most, the 5D Mark III and the 7D Mark II. The three other third party batteries are used in the 5D Mark II that I use when shooting with a three body setup, or occasionally with a two body FF setup when shooting bands or events in a very dark venue.
I normally use two batteries in a grip with the 5D Mark III and the 7D Mark II and can go all day shooting 2,000+ frames in either one without needing to change a set if they were fully charged when starting. At the end of a long day they usually still have 40% or more remaining. As the third body with the widest angle lens mounted on it, I normally don't have a grip on the 5D Mark II and rarely shoot more than a few dozen or, at most, a couple of hundred frames with it. I usually swap the battery out in it every couple of shoots or so when it is down to around 60%, but most of that loss usually comes from the long time intervals between when it is charged/used.
I also still have several Li-ion BP-511A batteries, bought between 2009 and 2012, for a Canon EOS 50D. They still take a charge and run the camera. But it has been years since I actually shot with the camera enough to have any idea of how long they would last. The 50D and BP-511A do not have a menu item to view battery recharge performance.
¹ Since I've never seen a third party LP-E6 show anything other than the full three green bars, I suppose it is possible that the third party batteries will display a full recharge performance value even if they are not performing any better than the Canon OEM batteries that show a reduced function. But my experience has been that quality third party batteries such as STK and MaximalPower will last more charge/discharge cycles before starting to show a slightly reduced capacity than the Canon branded batteries will.