This achievement requires you to COMPLETE 30 hunting days, which means kill 5 animals and get a final total on a hunt 30 times. Using the High Score counter, it will tell you how many total kills you have. You should be able to divide this number by 5, and that should be how many days you've been hunting, as I don't believe it counts kills from Quick Play or hunts that you don't complete.
HOWEVER, on my personal play through, I didn't get this achievements until I had 165 days, which translates into 33 hunting days. So I don't know if it's partially glitched, or programmed wrong, or the counter on the High Scores screen isn't 100% accurate. But you should get this achievement soon after your 30th hunt.