HAHA. I laughed when I saw this topic. I am literally doing the exact same thing you are planning on doing. Going into my junior year enrolled as a molecular and cellular biology BS (much less boring than a traditional biology BS, I would look into if your school has something like this) and general music BA.
All those horror stories you hear about music students having no life/2341231 classes are not true. At least, they won't be true for you if all you are doing is pursuing a BA. The crazy ones are BM students - bachelor of MUSIC. The requirements for that are MUCH more stringent, they have a lot more classes to take, and the recital/performance requirements are generally much higher. Granted, you are going to be taking a few 1/2 credit classes that meet for way more hours than they are supposed to, but if you love music it's really not so bad 🙂. Music classes can be a bit hard, so don't underestimate them. Theory 1+2, and especially EAR TRAINING can be killer if you don't have much natural talent to fall back on, but it can be done (I managed an A at least, heh).
I will add this caveat: I came into college with 50~ some credits out of the 120 needed to graduate, so I have had a LOT more time. I don't think I could have done it without the extra credits from high school. To do this kind of thing, you really have to look at your school's graduation requirements carefully and see whether you can even fit in all the classes in 4 years. After that, you also have to realize that you may want to take some research classes or join extracurriculars, so there needs to be space in your schedule for THAT too. Lots of careful planning is needed, don't just rush into this. If you do end up just doing music, I would recommend going for more than just a BA. Bachelor of music is just cooler because it is so much more performance based (and of course you have to take more theory classes, but theory isn't all bad). You will have to audition to get into a BM program though (much like the BA audition, just more focused on your technical abilities).
Here's what my schedule typically looks like every semester:
3 credits - private lesson (req'd every semester)
1 credit- concert choir (you will probably have to take an ensemble class like this every semester)
1 credit "convocational" - basically just a once a week, 1 hour long recital featuring students of various instruments/majors.
4 credits- theory and ear training (you will probably have something like this every semester)
3 credits music history (for a year you might have to have theory + history in the same semester)
4 credits biology + lab (I never took the second part of general bio...lol)
I also have a 3 credit translational research class, but that's not really a "class" in that I don't study for it, I just have an 8 hour time commitment/week. Private lessons are variable as far as time commitment goes. You can practice however much or little you want, but these are generally graded and dependent on how well you do on juries, and a 3 credit B is just bad no matter how you spin it. My school requires that you attend all recitals in your instrument, which means about once/twice a week you will have to attend a 1-2 hour recital, usually on a saturday or sunday. Your ensemble classes will meet for about 4 hours a week, but they are usually very fun (compared to high school where everyone sucks). Theory/ear training is just like any other class with a lab.
If you think you can handle 2-3 science classes on top of 10~ credits of music every semester (which will translate to probably around 20~ hours time commitment per week, including practice/rehearsals/recitals), some studying you'll have to do for theory/ear training, I think it's very doable. A BS in biology might be a little hard to swing because of the increased general education requirements if you don't have a lot of credits from high school, but if you put your mind to it I'm sure it's possible. Just make sure you don't overwhelm yourself and end up with a bad first semester GPA...
It all sounds like a lot, but in practice it's not nearly as bad as it seems. For the most part, if you enjoy music and playing the instrument that you've chosen, it's less of a chore than it is just something you do for fun. Good luck! 🙂
EDIT: I just read through MDWannabe90's post...Sounds like s/he's been talking to a lot of melodramatic music majors out there (which believe me, there are a lot...) Don't listen to the hearsay. Listen to me 😀