To add a time limit to your game, you can use the blocks in the Info menu.
Find start countdown, and add it to your on start loop, after all of the sprite set-up code.
While you are testing, it's a good idea to keep the timer short, but when all of your code works it's up to you how long each game lasts!
From the same menu, find the on countdown end loop - this is where you put your end-of-game code.
Look in the Game menu for the game over block, and add it into this loop. Click the plus button to choose a cool effect to take place!
Test it out - what happens at the end of your game?
If you are not happy with the default options, you can create your own Game Over screen by using other blocks.
For example, in the below example we have combined a splash screen block from the Game menu with a join text from the Text menu and the score from the Info menu!
Test out some ideas and see what you can create.