After mixing of the concrete homogeneously, you can pour it immediately into the structure, concrete should be poured into structure within the specified time i.e. before starting of its initial setting, concrete should be poured in structure.
Initial setting time of concrete is purely dependent on the design mix and atmospheric conditions.
Before doing concrete mix design, the parameter like distance of pouring location from mixing location is taken into consideration, type of structure, method of pouring, time required for pouring etc. according to the data collected, mix designing is done for concrete.