As A Rule Meaning

(idiomatic) In general; most often.

Example:   We go fishing every Sunday as a rule, but today the lake is frozen.‎
1907, Robert W. Chambers, chapter II, The Younger Set:
  His forefathers had been, as a rule, professional men—physicians and lawyers; his grandfather died under the walls of Chapultepec Castle while twisting a tourniquet for a cursing dragoon; an uncle remained indefinitely at Malvern Hill; […].
1918, William Strunk, Jr., The Elements of Style:
  As a rule, single sentences should not be written or printed as paragraphs. An exception may be made of sentences of transition, indicating the relation between the parts of an exposition or argument.