It depends on your objective. In my opinion, you can get very good estimate quite easily.
Since many [most?] of us tend to use little gas, I suggest the following...
Simply track all of your gas consumption (like you always did with a conventional vehicle). Then just apply the reasonable assumption that while running on gas (HV mode) you are getting 45 MPG. All the rest of the miles on the odometer are EV.
In my case, I have had the car for 7 months. I have consumed a total of 38 gallons of gas --> 38 * 45 MPG = 1710 miles. My odometer reading is 6039 therefore I have used 4329 EV miles and 1710 Gas miles (72% electric).
I also track all my electric input, and can do a sanity check that way too (expected miles per kWh).
This simple method seems like it is good enough for most any need I can think of. Not exact, but pretty darned good. You can always tweak the 45 MPG assumption by monitoring your actual MPG while in HV mode.