Concrete will attain a reasonable strength in three to seven days and will continue to strengthen over time. The concrete's curing process also affects the length of time that it can be exposed to outdoor elements. Curing times for common concrete mixes are as follows: Rapid-set Concrete - 3 hours; Standard-set Concrete - 4 hours; Slurry-set Concrete - 12 hours. The exposure time depends on the temperature and humidity outside and how often you need to check on the foundation. If you detect any signs of movement such as cracks or bowing, you should investigate what caused this phenomenon first before assuming the concrete is failing. In general, if there's no moisture on the surface and no signs of deterioration such as browning of the soil beneath the foundation, then the foundation should last at least 20 years unless you see cracks or other damage which would allow water in. You should call a professional when repairing concrete foundations because some problems may not be apparent until after they're fixed.