Differs for everyone, for me once a week. Can go once every 5 days but don't like the reduced sets it requires
Agreed.. And for example if you train chest & back on different days as biceps & triceps; in this case, the arms are essentially trained twice a week as there is much indirect effect involved... Even the chest gets involved indirectly when performing some triceps exercises...
After a brutal leg session; 7 days goes by really fast before its time to punish them again... More leg frequency may be indicated possibly in a case where a trainee wants to specialize to bring up the quads; but even then for only for a temporary period of time until this is accomplished...Once a week seems best for the most part, and will produce good results ...
Calves are a resilient muscle as we are on our feet all day... I feel they respond better to added frequency, and high repetitions to adequately stimulate the deep fibers... For this reason I feel they are better served to be trained twice a week..