The amount of time you’ll have to earn the required Fish Buffet Points (FP) to hit the next rank changes depending on the rank you’re trying to reach. In Fish Buffet, you move down ranks as well as move up ranks. If you don’t reach the required FP within the time limit of that rank, you will drop down one rank.

If you fail to collect the required points within the time limit, you will receive 10% of the cash value of the Fish Buffet points accumulated towards the rank.
​Example: if you have accumulated 1,000 points but could not complete the current rank, you will receive $1 in your account balance and your rank will drop one level and have 0 points for the new rank.

If you prefer to receive a fixed weekly cashback with a time limit of 365 days (unlimited time limit for Platinum Fish), you can opt for Platinum Ranks. You can switch to a lower level Platinum Rank or you can climb ranks to unlock higher Platinum Ranks.
​Example: While at a Shrimp/Crab/Octopus rank, you can choose to switch to Platinum Fish or you can choose to rank up to Whale and unlock Platinum Octopus.

You may find out more here.