Canon stopped using removable, user serviceable date/time backup batteries in the EOS line sometime in mid-2012.
The last cameras I checked that had one are the 5D Mark III and 1D X. Both released in early 2012.
The 6D released near the end of 2012 does not have one. Neither does the 700D released in early 2013, nor the 70D, 1200D, 7D Mark II, etc. that have been released since late 2012.
In the Rebel line the Rebel T3i/600D was released in early 2011 without a date/time backup battery. Subsequent models have all followed suit.
The Instruction Manuals for the various manuals have a rather nebulous statement that says it must be reset if the camera is stored without a battery or if the installed battery is exhausted.
I've let my EOS 7D Mark II go at least as long as a couple of weeks, maybe even longer, without a battery installed and the date/time settings were retained without any need for resetting them.
The beginning and end of Daylight Savings Time in the U.S. are the only times I can recall ever needing to reset the date/time of any of my Canon EOS bodies. Even my 50D has not yet exhausted the CR2016 that came already installed from the Canon factory. It has been stored with no BP-511A battery for very long stretches since about 2011!
I doubt the built-in capacitors in the newer models would last that long, but you never know unless you test it.