If you have not yet completed a degree then you could study a relevant subject e.g. maths, English etc, then complete a PGCE or alternatively you could study an Education related degree for primary school which would take three years then you would be fully qualified so to summarise:
3 year degree (e.g English) + postgraduate PGCE; or
3 Degree in Education
The former offers more versatility if you were tempted to change career
But at the very least, you should have a college diploma. basically this is four years.
You will also need a number of credits in education to fulfill state requirements. If you teach in a middle or high school also needed will be a number of credits in the subject matter that you wish to teach.
The amount of time that will take is dependent upon how you will be able to fit these courses into you BA degree and the qualifications for your particular state.
Some states will also require a masters degree or a certain number of graduate credits. Check with your school counselor or the School Board in your state for the requirements.
I would say that for most people it would be at least 5 years beyond high school.