Some people will tell you not to worry about break in. It's not the hemi you need to worry about, the engine in our Jeeps is more than strong. It's the differential that needs to be set properly. If you do launches early, you're basically screwing your differential and it will lead to further mechanical issues down the road.
The jeep manual says 500 miles / 800 km is what you need to do before being overly aggressive (ie: launches). It's fine to go fast up to 70/110 range before that in bursts etc but nothing like a launch. I'd say read your manual for sure on this.
Your time is going to come down to your altitude, your tires, your pavement. Higher altitudes are getting around the 4.5-4.8 range, lower are getting under that. Your jeep gets faster though as you add miles.
if you use the exact same everything, your 0-60 time will be faster at 5000 miles than at 1000 and even faster at 10,000 miles than 5,000. People with more mileage are getting better times because their differential has set properly. All SRT's are the same way on this, the hemi is strong but how it locks up is your weakest point.
Over time you'll see faster times though.
One last thing... Stock? What is this.... stock you speak of.
I've added a catch can and exhaust set up.. but before I did the exhaust, my best was 4.2
My blog with pix/vids - SRTgeek
2016 Jeep Grand Cherokee SRT Velvet Red
2013 Dodge Viper GTS - T/A Conversion Viper Bright White
2015 Dodge Challenger Hellcat Bright White Clear Coat
2014 Jeep Grand Cherokee SRT Bright White Clear Coat
2014 Jeep Grand Cherokee SRT Brilliant Black Crystal Pearl