When sending a staking transaction to the network, stake first needs to activate before it influences the consensus process and begins to earn rewards.
The time this takes is dependent on how much SafeCoin is beginning to stake relative to the SafeCoin already at stake. Up to 25% of the SAFE already at stake can warmup per epoch and start to earn staking rewards.
In the best case scenario, when a reasonably high percentage of SAFE is at stake and there is little new stake entering, this will usually mean that stake will become active in the upcoming epoch that the staking transaction was sent.
In times of high stake turnover, e.g. at network launch, stake will progressively activate meaning that only a fraction of stake will enter the validation stage each epoch.