Hmm, just checked my call log and it goes back to August 2013 (which is probably about the last time I wiped it). Now I'm on a custom 4.4.2 ROM, but the dialer seems stock and I've never found a mention of the call log being tweaked.
Of course it could be that there's a limit on the number of calls, in which case the time-span would be different for different people. I guess you could test that by seeing whether your oldest calls disappear as you make new ones.