There aren't many rules of thru-hiking. There's no official guidebook to spell out the correct way to complete a long-distance hike. Purists might claim a set of standards must be followed, but most hikers do well just to live by the old trail maxim, "Hike your own hike."
The closest we have to thru-hiking rules are the Leave No Trace principles. Federal agencies and trail organizations adopted them to protect the outdoors and ensure everyone can enjoy it safely.
Other than these, I can think of only one generally accepted rule for thru-hiking, and really, it's just a way to define the term. The Appalachian Trail Conservancy and some other trail organizations say a thru-hike is a hike of the whole trail in 12 months or less. If your hike takes longer than that to finish or you don't intend to hike all of it, you're not a thru-hiker. You're a section hiker.