I'm a big fan of platform games, such as the Mario series, the Sonic series, the Kirby series... There's something so very simple about them. You appear in a location, you must reach the exit, the end... There's lots of traps around, and enemies roaming the place, and dangers to overcome. Occasionally, there's also the ever-popular endless pit, guaranteeing a instant death to the player character. Some games like to spice it up by adding a puzzle aspect; as an example, you need to find switches and push them in order to access other areas of the level as well as sometimes the exit. In other cases, the game also includes a collecting aspect and you must find an amount of objects in each level. Basic enough. You could say that platformer games don't usually have a big plot. You could be right; ypu could be wrong. Depends on what platforming franchise you're playing. Usually, a platformer means two-dimensional side-scrolling levels.
However, the size of the levels is all up to the designers. Short and sweet, or long and tedious? Again, it depends on the franchise. There are franchises who like to keep it to simple levels, others try to make "marathon levels". This term describes levels that are so freaking long that you need many minutes before actually finishing it. Some games use and overuse those "marathon levels". Of course, each gamer has its own opinion on the matter. Are overly-long levels a good or a bad thing?
One of the strengths behind the platformers of the old consoles was that there was a time limit. Literally, if the timer hit zero, it was an instant loss. As if that level you just roamed had stopped existing with just about everything inside it, until you tried it again. Of course, the levels had to be do-able within the time limit, which ranged from 200 to 400 seconds. I'm going by the first Super Mario Bros. game's standards, though. The longest level in that game was 8-2, and more often than not you'd cross the end flag with the time limit below 100 seconds. The predecessor to all marathon levels was born. The games came and went, the consoles appeared on our shelves and vanished years afterwards. Platformers became common in video gaming, until the perfection of the 8-bit 2D games was beaten by the almighty three dimensions. Now, you could roam open worlds, sometimes choose what you were gonna do. Other times, the levels were designed so you could know where to go. Or, in some cases, you'd still have to search for where exactly to go.
One good thing about the shorter levels (say, five minutes and less of length) is that while it is true that you spend a few minutes reaching the exit, at least it was reached quickly. If you had to go to the bathroom, usually you were able to finish the level before leaving your seat. With marathon levels, forget about that. Those levels, I'd say they are five minutes or longer. Yes, you could pass checkpoints to continue at one part of the level instead. But in the end, when a level is too long, the player notices it.
How often have I played a level and told myself "Where the Hell is the goddamn exit in this level???" If you're asking this question, then the level is way too long. And that's a problem. Marathon levels are so painfully long that, after a while, you lose interest in a game. No, really; having such long levels, through a whole game, can get annoying. I know I do. When a level is too long, I kind of lose my focus on it, and in the end I seem less interested into completing the level and more into FINDING THE DAMN EXIT. And yes, this often happens through an entire game. The 3D Sonic games have been guilty of this. Think of Sonic 06's longer levels; they take over ten minutes for you to finish them. When there are puzzle elements added, it gets worse; to find the exit in that level, you might spend many more minutes searching for the way to reach the exit. A series especially guilty of this is the Drawn To Life franchise. And yeah, when you don't have fun anymore with a level because you just want to find the exit after a while... That's bad.
Marathon levels? No problem. As long as it's used once in a while. A marathon level can be fun, but when it's the only type of level in the game... it's tiring. You just want it to end. Like many other video game features, there's a balance that must be gained for the game to be enjoyable for the players.Too many levels that are too short, too many levels that are too long... Both are bad. The developers always have to find the right balance, the right size of levels... and sometimes they make the mistake of making levels too large. It removes a part of the fun. But we can't blame them, either; it's not easy to develop a game.