If they are vacuum packed they will last at least 3 years. If they have been opened, then you need to use them within 4 to 6 months. After you open the vacuum pack, put the rest of them in a ziplock freezer bag and get as much air out as you can.
You'll probably use them in about 6 months. I would imagine that within 6 months you will need to switch it to fuzzies, then after another 3 months or so to hoppers. By one year, it should be eating large hoppers or rat pups.
Typical diet for baby kingsnake:
First feeding through 2 months - 1 pinkie every 3 or 4 days (appr. 20 pinkies used)
2 months through 4 months - 2 pinkies every 3 or 4 days (appr. 40 more pinkies used for a total of appr. 60)
4 months through 6 months - 3 pinkies every 3 or 4 days (appr. 60 more pinkies used for a total of appr. 120)
6 months through 9 months - 1 or 2 fuzzies or 1 or 2 rat pinkies every week
9 months through 1 year - 1 or 2 hoppers or 1 or 2 rat pups every week
by 14 or 16 months it should be feeding on small adult mice or larger rat pups.
The first year and a half is when they really grow fast, so you need to feed them plenty during that time. After that, you can cut down to one mouse or small rat a week.