In my experience if they are real beginners they`ll take around 6 to 12 months, depending on the amount of time they study.

If they have at least 3 to 4 hours a week they reach a good level in 6 months. Though they need to practice a lot at home and get involved with the language asap.

Apart from class, make them practice with music and movies at home, that`ll help a lot.

Motivate them with things they like, so they get interested.

Good luck!

Most in depth language studies (to become fluent) take around 2 years, however, immersion in the language will really speed that up! I would recommend that you set a certain number of hours per day that NO SPANISH is allowed — only English. My parents learned the bulk of a 2 year language course with this “immersion” method in about 4 months. Granted, they weren’t perfectly fluent, but they could carry a decent conversation and buy groceries, etc. I was a young child at the time, so I learned almost the entire language in about two months. So, also, if your students have kids, also immerse the kids and tell them to speak English as much as possible in their own home. My parents had every meal-table conversation in our other language, since all of we kids were already fluent. It really helped them learn fast!