I said "long" because I don't like "short", but honestly, I prefer "medium". Give it a reasonable form, minimum of two verses, sometimes three, one chorus per verse, plus one instrument break and one extra chorus. Mix in whatever order you like.
My favorite forms are
ABABaCB, ABABCaB, ABABaCA'B, ABACBabACB
Where A is a verse, B is chorus, a/b are instrumental verse/chorus, C is a bridge, and A' is a half verse.
That last form is about seven minutes for a ballad. The first is about five. The others are in-between.