It's a matter of preference since there is no strict cut-off. Many people will follow their own surname through previous generations, or a small group of surnames, although surnames are not really significant from a genetic or lineage point of view.
I follow both surnames at each generation (i.e. paternal and maternal lines), and basically go back as far as records allow me to. For each of those lines, I also project fowards in order to find the siblings, cousins, and related descendants. In effect, each ancestral line is a family tree in its own right.
This may sound like a huge task but the advantages are in finding "distant relatives" who may be able to help with missing history, important citations, copies of documents, photographs, etc. Since I'm researching family history, rather than just genealogy, then those extended families can often be very significant.