If the meat is frozen, you can cook it on low for 8-10 hours. If it’s thawed out 6-8 hours on low is fine. If you need to speed things up a bit, cook it on high for 4-6 hours.
I can honestly say that I normally start my crock pot at 7 in the morning and don’t return to it until about 5. In nearly ten years of adult life, there’s only one recipe that has ever burned on me. So I don’t get too worried about crock pot times. Just let your delicious fajitas slowly simmer the day away.