I agree, no more than 1/3 of the plant at a time....the smaller the plant the less I would prune. Each plant is different and also the size, growth rate and health all are very important factors to include. I like to do it in stages not shocking the plant just before flowering. Often I continue to prune lower small branches on larger plants as they flower, until the canopy is filled in I give the lower branches a chance make a decent bud. Also pruning the lower branches promotes vertical growth where topping is the opposite. The plant would do better if you topped one day then prune another. Like BeastGrow says ".... I prefer to do minimal pruning". I totally agree, every leaf makes a difference and those leaves make the sticky buds, so prune only what you need to.