Admission officials typically expect two or three years of foreign language study, and the most selective schools are often looking for fouryears of the same language. So if you started taking Spanish in middle school (or even in elementary school) and were thus ready for AP by grade 10, you can get away with quitting your language classes at the end of this year.
BUT … if you entered Spanish 3 as a high school freshman without taking two previous years … because you are a native speaker or you live(d) in a Spanish-speaking country or you learned Spanish in some other way outside of the classroom, then it's not such a hot idea to quit your language study when AP Spanish ends.
Admission officials—especially at the most sought-after institutions—can be a bit picky about whether a language was actually studied rather than mastered through experience. Granted, one might argue that it shouldn't matter where you learned the Spanish that led you to AP. But those college folks are sometimes snooty about language prowess not acquired in an academic setting, even though that sounds a bit silly to “The Dean." Fluency gained by living abroad, stocking shelves in a bodega, or spending vacations with Abuela in Alicante can provide a cultural context for the language itself, and thus the value of time spent in a classroom seems largely overrated.
Bottom line: If you apply to colleges … even the most hyper-competitive ones … having completed AP Spanish but not having spent four years in high school language classes, it won't be a deal-breaker but it could be a small liability. If, however, all your Spanish language know-how came from the classroom and not from the dinner table, college officials are more likely to approve.
Yet, if you were my child and already taking AP Spanish in 10th grade, I'd probably encourage you to continue … either via classes at nearby colleges or over the summer. Not only will this guarantee that you will meet all college requirements but also—and most important—it will help you to develop or maintain communication skills that may prove helpful throughout the rest of your life.