1 credit = A unit of credit may be defined as being equivalent to at least 130 clock hours. Some parents consider completion of a course book as being one credit irregardless of the time a student takes to complete it.
Keeping a list of what actual skills were learned is another way to give credit; Home Economics can be learning the basic skills needed to run a home -- just record what those skills were.
Whatever method you use, it is probably a good idea to note it in your records (ie. # of hours, days, or material).
Credits are earned by the .25 (one-half semester or one quarter school year), the .5 (a semester or one-half school year), or in whole units (one school year).