Example:
Right now according to
https://docs.google.com/spreadsheets/d/1Y8uRsftL_uUcDPPiRvDDxJwI2ftLZq1iPavKaGfrF_c/edit#gid=0, the staking ROI per year is ~14%.
If you have 100 BLOCK, you should get 100 * 0.14 = 14 BLOCK in stake rewards per year.
(365 days / 14 BLOCK) * (0.3 BLOCK / Stake Reward) = 7.8 days / Stake Reward on average.
Note that this 7.8 days is not constant; it is just on average. You may get lucky and find two blocks within minutes of each other or get unlucky and not find any block for a while.
@EnViRoNmEnT: you mentioned you waited 2 days and didn't get anything. According to the above calculation, you need to be staking at least 400 BLOCK to get your first 0.3 BLOCK stake reward after about 2 days.
Forgive me if it has already been covered (probably 100 times already), but I can't find out how to start staking?