Fresh concrete should be protected from freezing for the first 24 hours after placement, by which time it should have a compressive strength of 500 psi. After that, the amount and duration of protection will depend on the desired rate of strength development.
Generally, the greater the cement content and the section thickness, the greater the heat of hydration and thus more rapid strength gain of the concrete. You can use the maturity method from ASTM C 1074 if you want to correlate the temperature-time factor of the in-place concrete to its strength.
Other references that may be of interest are ACI 306, “Cold Weather Concreting,” and the first section in ACI 201, “Guide to Durable Concrete.” Beyond that, the 14th edition of “Design and Control of Concrete Mixtures” gives a maximum saturation of capillary pores at 91.7% filled with water as the point at which concrete with or without air will not be effected by freezing and thawing cycles. The 9% expansion of water as it freezes has the room for expansion it needs to avoid disruptive internal pressures.
Another good resource on this subject is T.C. Powers’ "Prevention of Frost Damage to Green Concrete," which is available as a free PDF file at www.cement.org/bookstore (search for "frost damage"). That is the reference for the 500 psi minimum for a single frost cycle in ACI 306. Remember that the re-saturation of pores requires very little water from an external source to render a non-air entrained concrete vulnerable to damage.