The Piece of Cake event is from February 21st–26th (+7 days of rewards). What that means is your world has from February 21st to the 26th (server save) to complete the event. If you don't complete the event, you won't get the rewards. To get the rewards you need to wait until the server save after your world completed the Piece of Cake event for NPC to appear on the Cake Island.
Once Brutus The Baker appears you will also have the experience and regeneration bonus for 7 days and he will remain on the island for these 7 days. After these 7 days of rewards the bridge to the Cake Isle will disappear and you won't be able to get to Brutus The Baker for the rewards.
By completing the event I mean your world needs to eat the whole cake, you in particular don't need to do this part to get rewarded. However, you do need to visit the cake extractor machines in order to get the rewards and you need premium account as the Cake Isle is in Cormaya.
Please see these two links for more details on doing this cake isle extraction machine task if you need help:
Please see this link if you want to know what the rewards are:
Ultimately, it all depends on when your server has completed the event! For example, If your world completes the event on the 21st, the next day on the 22nd server save Brutus The Baker will appear for you to claim your choice of rewards and so will the regeneration and XP bonus. Your world will have 7 days of this bonus starting on the 22nd's server save. 7 complete days would mean it will last until the 29th server save if there's 29 days in February, if there's only 28 days in February and then the rewards will last until March 1st.
The way you can count when an event will end is to be reminded of rapid respawn or xp/skill events they start on the server save of Friday typically and then end on the server save of Monday which is counted as 3 days. Now for events that last 7 days a good trick is to look at your calendar when it first started on that server save so let's say Saturday's server save then you'll see that 1 full week will be on next Saturday's sever save.
I'm going to paste an official reply from Tibia's forums, please note in this answer is in regards to if there's 28 days in February and not 29 days like it is for the year 2020:
The access to the cake island gets removed after the reward time (increased mana/health regeneration and 50% XP increase for premium accounts) has run out.
Most game worlds completed the event on February 22, which means that the bridge should disappear on March 01 [server save] on those worlds. [This answer would apply if there's 28 days in February however if there's 29 days then it will disappear on February 29th server save]
The rewards and therefore the access to cupcakes is not limited to those who actively participated in the event but available for everyone who helped Baker Brutus to collect the recipes from five different cities. This is working as intended and the reward can be collected as long as the cake bridge is still open.