I usually do a couple leaves, and a stem or two a day over a course of a week. I've done large trim backs all at once and that was the only time I've noticed any slow down in growth. Honestly the whole trim vs don't trim is a raging argument within the growing community. So guys absolutely don't believe in trimming anything other then stuff that is dieing off like your currently experiencing. Then others will do large defoliations like Neon said, some times multiple times per crop. I follow a path somewhere in the middle, I feel there are benefits to be had particularly in indoor growing with limited/ controlled conditions. Air flow, and light peneration being the largest of those benefits. If the foliage isn't getting any light then it is living off the rest of the plant in my mind. Not that a plant isn't more then capable of growing more then just the colas, but in our game we usually want the best quality we can get. Focusing the growth towards the parts of the plant that are actively getting light and producing seems logical to me. Its just my 2 cents, I'm not a pro but I've got more then a few years under my belt and have always had great success.
Peace TC