It really depends on how far down the rabbit hole you want to go. There are a number of luthiery schools in America, many of which have 3-4 year programs that are similar to going to college. The North Bennet Street School in Boston, the Chicago School of Violin Making in Skokie, the Violin Making School of America in Salt Lake City, and the New World School of Violin Making in Presque Isle all have 3-4 year programs that typically involve some variation of making 3-4 violins, 1-2 cellos, and 1-3 "instruments of your choice" which can be basses. Some have a bit more flexibility than others, but typically focus on violins because the material costs and time needed to complete violins is a lot less than basses, and many of the tool skills and concepts translate fairly well. They also tend to focus on things like technical drawings and acoustics in a very academic fashion with classes and exams.
While none of these schools are specifically bass making schools, (I don't believe there is such a thing out there, but I could be wrong) a number of them have faculty that are very comfortable teaching basses, and students who successfully go on to make great instruments. Eric Rene Roy from Upton mentions attending the North Bennet Street School in his TalkBass profile for example.
On the shorter scale, there are various workshops as well, typically offered in the summer. There is a program at the University of New Hampshire that has a violin making course that runs up to 4 weeks, with the idea that you attend several consecutive years in order to complete the initial instrument. They also offer various repair and setup courses as well. The VSA is connected to courses run through Oberlin College, and they have a Bass workshop that a number of people on here have attended, and I believe a few have been on faculty as well.
While attending a few weeks in the summer can give you a good foundation, and many people go away from these courses able to perform various tasks, a number of them return year after year to learn new things and hone their skills. It can be a more financially manageable way to pursue an education as you typically spend $1 000-2 000 for a week including tuition, accommodations, material costs etc. where the 3-4 year programs can run anywhere from $3 000 per semester to $24 000 per year, not including living expenses, and tools and materials in some cases.
Although rarer than it once was, some makers will take students/apprentices for varying lengths of time. A few offer "courses" where you can pay to study with them for a set length of time focusing on specific skills.
What you would like to learn is also a factor. If you're looking to re-glue seams, close minor cracks, cut bridges and fit sound posts for example, that can be learned in a shorter amount of time than learning how to do large scale restorations or make instruments.
So depending on how far down the rabbit hole you want to go, you can look into various schools and their admissions requirements, look into summer workshops, or contact makers directly. Having a clear goal of what you would like to learn is definitely an advantage at that stage. Having even some basic knowledge about hand tools doesn't hurt either.
