Behaviour of normal OPC based concrete with time is known and available in the literature likewise its probable strengths in 7, 14 and 28 day, respectively. Once, you know how other material parameters influence the concrete strength (action), you can figure out the relation of stress with time but such equation cannot be used in isolation for what concrete means. No single equation can fit concrete stress with time, as we know concrete being a static and complex heterogeneous whole. Most important equations are those that relate concrete material parameters/properties (w/c ratio, s.g of the aggregate and their proportions, sand/cement ratios, fineness and surface areas of the materials etc) to its mechanical properties such as the elastic modulus, tensile stress, modulus of rupture and ductility (M/EI).
However, if you are interested in concrete rheology or workability properties, you may be inclined to determine the rate of flow (dynamic property) and how such relates to yield stress and minimum energy required for a full compaction of concrete mixture.
There is extensive literature on the strength of concrete over time. Concrete is made for different purposes so has different characteristics with time. In general, concrete achieves 90% strength in 4 weeks and maximum strength in 25 years, declining slowly thereafter. Many engineering references are available, including the internet.
refer to ant text book on concrete technology which will show details of hydration and hydration products like C2S,C3S,C3A, C4AFcalcium silicate hydrate (C-S-H) .I do agree with Joseph Alvarez regarding strength development.
You can find a general model to predict of concrete strength in due time, in a reference book about concrete. It is Concrete properties by Professor Novill.