Whether or not you received college credits in highschool, whether the two degrees you're pursuing have overlapping courses, your willingness to load up on credit hours in a semester, do summer courses, whether you need to retake a class or take a semester or two off for a co-op; there's a lot of variables here.
The absolute best thing that you can do for yourself is speak with the counselors provided to you by your school. My school's college of engineering had a few general counselors available to help guide you to useful resources in the school, and when I chose my major that department had an additional two counselors who could help you work through both your academic and career goals. So develop relationships with these types of people, speak with them early, they'll help you make an academic plan for the next few years, and then it's up to you to execute.
With all of that said, the most-likely answer will be about 5 years for a double major if you take a few courses during summer semesters.
If you are short on these "Advanced Placement" or "College In the Schools" options, you could take summer classes at a local community college and transfer those to your 4-Year College or University.