Getting back to training either 1 or 2 times per week now; I'd say one, but that is just me. The way I tell people to decide when asked this, is basicly to just do for calves, what you would do for your other muscle groups, assuming it is working!!! If you do Chest, Back, & Delts once per week, and you are getting results, then stick with it, and train all other muscle groups the same. Abs too. Sometimes, adding more intensity during sets(and by intensity I mean forced reps) equates to better results than adding an extra training day per week. This way, you are increasing the amount of work you are performing while training said muscle, while at the same time not sacrificing rest.
Good luck