You can calculate credits by time spent on task or by amount of work done.
If you want to use the amount-of-work-done method, I suggest you check the amount of work involved in your school district’s one-credit course and compare. If the map work and living book assignments for older children in the six Visits to books amount to half or more of what is covered in the district’s course, you could assign one-half credit for completing the six books.
If you want to use the time-spent method, average how much time your student will spend on completing the visits per week and multiply that out for the entire school year. A full credit usually is awarded for 120–180 hours of time on task. So you would need at least 60 total hours of work for one-half credit.