The amount of hours of classes depends on the student. If you are just getting started, make sure to take at least two weeks of classes. That way you will be able to speak enough Spanish to strike up a basic conversation, order your food and ask for directions.
Even if you want to take more classes, start by reserving two weeks at the Spanish school. Spanish schools always welcome students who wish to add more classes afterwards. They will usually ask you after the first week if you want to take more classes after the second week.
By starting off with just two weeks of classes, you can choose to take more later or change schools if things don't work out. It is a lot more difficult to cancel classes once they have been reserved.
But, I already speak some Spanish
If you already speak some Spanish and you would just like to learn some more in Latin America, great! It will save you some time and money.
Take some classes to learn the local or national slang and work on your verbs a little bit. Start with a week and explain to your Spanish school that you already speak Spanish (a little bit). This way the school can assign a teacher with experience teaching students with knowledge of Spanish.