As previously stated you become a Diamond Plus after you complete 24 cruises. On your 25th cruise you are officially a Diamond Plus member. After your 20th cruise you should have or will receive a crystal block with the ship incased in it representing the ship you were on for the 20th. It will be given to you in your stateroom. Each 10 cruises after that you receive a block.
I was just told recently (when we received our first block) that since the points have changed it is now first block after 130 points and a new block every 70 points thereafter. So you can no longer go by the number of cruises because the 3 and 4 day etc. only count for 3 or 4 points instead of a whole cruise. The new structure is Diamond Plus member at the 175 point mark. (I'm only 8 away)