orchids always seem to me to grow better and faster with a pot full of friends....BUT
if its out of shape and growing out of the pot and needs to be controlled it just needs re potted.
if its overgrown and the center is fallen out and started to decline, thats a good time to divide.
usually orchids grow from the center towards the outer rim of a container. interior plants at some point will start to decline and thats when to divide IMHO and most often will fall apart when you take them out for inspection