Normally curing should be started immediately once the concrete is initially set which generally occurs around after 2 to 3 hours of production of concrete.
As the construction keeps on going contentiously it is always not possible to cure the concrete for longer time till 28 days.
but we should cure concrete for minimum;
in case if concrete is produced with only OPC cement - it can be cured for minimum 7 days.
In case concrete is produced with OPC cement and GGBS - it should be cured for minimum 10 to 12 days.
in case of concrete is produced with OPC cement and flyash - it should be cured for minimum 14 days.
If the conditions allows you to cure it for 28 days, its well and good. If not then, you should ensure above minimum curing period for concrete.
Yes sir absolutly right depends upon climatic condition and usually within 24 hrs.. As far as Formwork intact moisture inside the foundation intact.. Yes I agree if surface area of open area is more than no doubt.. Like concrete slab I agree