The basic rule of thumb is 1 credit = 150 study hours.
Rationale typical high school class is 50min x 180 school days = 150 hours.
This formula is more necessary when creating your own curriculum, using unit studies, etc. When using a purchased curriculum and not creating your own basically you go through the textbook lessons, do all assigned activities, complete any tests and quizzes in the curriculum. When you finish all the material, you have earned credit for that class.
Parents do a disservice to their childrens’ education if they “just make it up.” Most homeschool parents choose homeschooling because they want to provide their children with the best possible educational opportunities and will not arbitrarily give their student credit unless they have actually done the required work and mastered it to their satisfaction. Sometimes the parent is a much stricter enforcer than the public school. My younger son does not receive credit for a class unless he completes all material with an 85% or better average. My older son must master all material with a 90% average. If they complete a unit test with less than their required mastery, then we erase all their unit work and they restudy everything from the beginning and take an alternate unit test.