Purging is continual with me. I'm a Lumper (big picture and similarities vs details and uniqueness a Splitter sees). So I can often decide that items in a category are not working for me and I will get rid of them all.
I don't have rules like one in, one out. Though I don't want to be overrun by clothing and, since I like to hang things, when I see I'm running short of hangers, I am mindful to be purging, since I would rarely be willing to buy more hangers (my sign of being overrun by clothes)
ETA: I try to shop as little as possible out of season because I'm weather sensitive. And I try not to purge too much out of season. For one thing, the light conditions change! Winter season here is extremely overcast. Summer is mostly bright and hazy.
I purge a lot by color. I want the item, if a top piece, to light up my complexion. However, recently have become aware that light conditions matter and some colors work well on me in overcast weather and others in sunnier weather. If an item needs something else to run interference for it - different color nearer the face - then I'd rather get rid of the item.
Usually, whatever newer items are added can be a signal of a newer direction and I will then be looking to purge what I am moving away from.
I'm very experimental and open-minded about clothing. I shop thrift mostly, because that allows me to do that without running up against a budget. Lighting is thrift stores, and stores in general, tends to be poor or distorting. Result may be that after I get something home, it just won't work. Or, because I do my own alterations/remodeling, and can't picture how something will feel on *after* the alterations, I will go ahead with them and if it isn't fitting comfortably the way I want, it will go. So some things go without being worn or more than once or twice experimentally and other things I wore all the time but am sick of.