If the players have just rolled up to a major hub city for the campaign and you really need to get the description out of the way in a hurry, you might have a much longer description that includes three different shops, four different important factions or major NPCs, and a half-dozen major threats intended to keep the players occupied for the next ten adventures. Most of the time, however, you’ll either be presenting a simple situation, in which case devote one sentence to the room and one sentence to the important monsters/treasure/NPCs in it, or you’ll be providing the players with an update to a situation, in which case you can probably get away with a sentence fragment if you really want to shine in the word economy department. When presenting a situation, your goal should always to make it as short as possible while still describing every important thing in the scene with one and maybe two distinguishing details. When rolling up to that hub city, for example, if you can spread the description around so that only one or two new shops, factions, or major threats are introduced at a time, do so. Players want to interact with your world, not just listen to you describe it, so only resort to long descriptions or narration when you can't find any other way to deliver important information to the PCs by the time they'll need it.