So you've designed a level and don't know when to stop? Or maybe you think you will end up in that situation at some point in the future. Fret not, it's fairly simple, and you can make any level memorable in the process!
The Designer's End
Once again, I will first mention when you should stop the level. Now that we have infinite checkpoints, the answer is basically: Whenever you feel like you have enough, or when you run out of ideas. Short levels are nothing to be ashamed of, and neither are long levels. What matters is that you did everything you initially set out to do, and maybe some extra things you have come up since. Essentially, as soon as your level provides gameplay to the "answer" from the previous thread, it can be ended, but you can keep going for as long as you have ideas that expand upon that answer and perhaps make it even more exciting to play.
I will continue using the Pyramid from last time. In that level, Mario will make it past many cunning traps, like the classic dart traps as obstacles replacing Bowser Statues, fake bits of ceiling that are thwomps with a wider warning radius than normal, Red Donut trap doors, Coins that, when collected, cause the floor to sink towards spikes... The list goes on for as long as you can think of more traps for the player to maneuver past in interesting ways. But it doesn't yet provide gameplay to the answer: Neither has the player escaped the pyramid yet, nor has he seen any treasure. These two things happen to line up super well for a potential climax to the level! And once you have the climax and don't have anything else to explore, the level can be done!
You can even make the climax first and just put a warp to it later on when the rest of the level is built, since, especially with the pyramid, the 'first half' of the level is what's most expandable. In many cases, though, it makes sense to design the climax at the end, for reasons I will get to in the next segment.
The Player's End - The Climax
What I dislike most about levels is when they put a door after the 2nd half has gone on for 2 minutes and behind it there's just a star or an end. It's like, "okay, I guess the level is over now". It leaves me wanting more, a cruel cliffhanger to an otherwise perhaps excellent level, as well as the motivation for this series of threads.
So let's talk climax.
The climax is the "sendoff", the proper ending to the little story you express in the level. It can be a shift in gameplay, or just more of what was established, or both. I'll write up a few ideas further down, but first let's imagine the boulder chase from the pyramid: After collecting the star, it turns out to be a fake (exit NPC, not fake treasure), and a boulder crashes down behind Mario, revealing an opening to go through that leads into an autoscrolling section where traps need to be avoided while the player is chased by the boulder. It's a final test on what the level has done so far, with a twist that adds more pressure! At the end, Mario is ejected from the pyramid, the treasure drops down next to him, and when you pick it up, the level ends.
You'll often hear me and others talk about this as the 'final section'. Some sort of spin that aims to create a memorable moment or prepare the level for ending. This is often where the coolest part of the level happens, since the player is now educated in everything the level set out to do.
In the farm level with bouncy sheep from last topic's spoiler tag, there is no such internal conflict, so a final section would instead feature some unique platforming challenge. Perhaps there are flying bouncy sheep, and chickens have to be tossed into cubbies to open gates while you try to platform across these sheep over a bottomless pit. It would be an elongated 'unsafe area' with no proper resting spot, similar to the boulder chase. And when it's over, if it was done well, players will say "man, that was awesome" to themselves, and will expect the level to end.
Climaxes come in all shapes and sizes. Boss fights, enemy gauntlets, crumbling structures, autoscrollers, fake CP1 contraptions, etc.. If the final section is the epilogue, then the climax is the part where the villain is bested. Ending a level before it is like ending Lord of the Rings when Mordor is entered. And as a BONUS, they also create some way for people to remember the level for a long time. Because a level is a story, and just like with boss fights, the ending is always the thing people will remember the most.
Hope this helped some of you. Keep creating awesome levels!