*Why colostrum is crucial
Colostrum is produced by the cow up until the time of birth. Calves do not receive antibodies from their mothers in the womb (unlike human babies) and must have early feeds of colostrum to receive the antibodies they require to thrive in the environment into which they're born.
The antibodies from the colostrum are absorbed directly into the bloodstream from the calf's intestines, but only for a limited time. Some sources insist that the time-frame is only during the first 24 hours, others suggest the ability can continue, to a reduced and decreasing extent, for up to 36 hours.
If a calf never fed in the first couple of days, obviously you'd still give it colostrum when you found it, but if you're watching a calf which hasn't fed in the first few hours after birth, it's best to get in and help within 6-12 hours and make sure it has another lot within the following 12 hours. You probably won't get more than a couple of litres into a newborn calf at a time, so go back three or four times to make sure it gets the best start possible.
Colostrum can be frozen for up to a year, so keeping some in the freezer is a useful backstop. Always thaw it carefully to avoid damaging the antibody content, which means slowly and gently in a double boiler on an element (so it doesn't accidentally get scorched), not in the microwave, and don't let it get too hot.