After you have finished delegating your stake, it will become "live" right away but not yet "active". It will become active after the current and the next epochs have ended. An epoch is a period of 5 days in the Cardano network.
When your stake is active, you will be entitled to rewards for blocks produced by the stake pool. Supposing the pool minted blocks in that first epoch when your stake became active, you will receive your rewards for that epoch after 1 more epoch has passed.
For example, if you delegated in epoch 220, your stake becomes active in epoch 222. Supposing the pool minted blocks in epoch 222, you will receive your first rewards in epoch 224. For blocks produced in epoch 223, you will receive rewards in epoch 225 and so on, every 5 days.