OLI offers Spanish independent study courses to students for $10. While there is a gradebook with graded assessments, there is no teacher or schedule. The course is completely self-study. 
OLI offers no credit or certification of any type. Any arrangements for credit would have to be made between you and the school where you want to gain credit. Since OLI does not offer college credit, any arrangements for college credit would have to be made with a college that does give credit. If your main priority is high school or college credit, then you may want to start with the high school or the college and ask what their requirements are for granting credit (instead of starting with the materials since OLI doesn't offer credit).
To register in the Open Learning Initiative's Spanish - Self-Paced for Independent Learners course(s), follow these instructions. The cost is $10. Sign In with an existing account at oli.cmu.edu or create an OLI account here: https://oli.cmu.edu/jcourse/webui/signup.do

On your My Courses landing page, enter the course key in the "Register for a course" box and click GO.
Spanish I Course Key: spaniself
Spanish II Course Key: spaniiself
Enter payment when prompted via credit card, debit card, or Visa/MasterCard gift card.
If you choose Spanish I and decide you would prefer Spanish II (or vice versa), just let us know and we can switch the enrollments. Also, let us know if you would like a refund. Email oli-help@cmu.edu or click Help from any OLI page.
When returning to your course, sign in to OLI here: https://oli.cmu.edu/jcourse/webui/welcome.do
OLI offers these courses to independent learners. The self-guided courses are completely online and wholly contained  within the online materials. OLI does not provide instructors. OLI helps with any technical questions or basic questions about how the course is organized and accessed, but OLI does not support the Spanish content. There is no set timeline, you can choose to cover the materials as quickly or slowly as suits your schedule and choose to block out or schedule your time as works for you. There are built-in practice activities on the pages and automatically graded tests at the end of each lesson (if you want to take a test over, just let us know and we will delete the test so you can take it again). The independent study courses have no credit or certificate.

Barring any unexpected changes to the OLI platform, there is no time limit to your access to the OLI course.