I doubt this is physically possible 
Maybe it’s semantics: a “fret” in this discussion is the distance between two fret wires, i.e., the place where you’d fret one specific note.
You should be able to span at least two frets (i.e., two notes), and fairly easily three frets (i.e. three notes, each a half step from the next).
Beyond that, you will find people who can span 4 (and sometimes 5) frets and will often play like that. Most others will typically span 3 frets from index to pinky, and either stretch a bit more if necessary or micro-shift).
It’s important to note that doing micro-shifts is not “inferior” or like a handicap; in fact, even if you could span a larger range of frets, micro-shifting can be advantageous to re-position your hand for the next notes you will have to play in a certain bass line (hope this makes sense!?).
I’d study videos of other people playing and see how much they stretch and/or micro-shift. Head over to the “Post Your Covers” thread and check out how other people attack this “issue”.
You can see me here micro-shifting ALL the time