What is the length of time required by law that you need to be in a house (as your primary residence) before you can sell it?

There is no law regulating your stay in a primary residence before you can sell it. To have your gain on the sale be tax free, you have to wait two years. On most Owner Occupied (primary residence) mortgages, there is some verbage in the paperwork, that you INTEND to remain in the property for at least 12 months. This is to discourage people from using Owner Occupant (at a lower rate than Non Owner Occupant) mortgages to build a portfolio of rental properties. This verbage has no bearing on you selling the property after a short time. I hope that this covers your question.

For tax requirements:
Any gain on the sale of the home over $250,000 for single must be reported, $500,000 for married.
You have to of lived in the house for 2 of the last 5 years to not have to report any gain under $250k ($500k married) on your tax return. So if you lived there 1 year, rented it out for a year or two, and then lived there another year you would still meet the 2 year requirement.