When they are worn basically.

But because this was my very first set of cleats I'd no reference point as to whether my cleats were worn or my pedals were knackered, or both. I tried tightening the tension on my pedals a touch, but my feet were still pulling out of the pedals when climbing. In an amazing, accidental, act of foresight I'd actually purchased a spare set of cleats when I'd got my shoes and pedals, but because my cleats had been on my shoes for just over a year and a half I was apprehensive about being able to get the old ones off without damaging my shoes.

However, as can be somewhat ascertained from the photo in the linked article, the only way to tell if your cleats are worn is to take them off: they can wear as much as possible on the side that touches the floor and it doesn't impact their ability to work at all, what matters is the length of the underside of the cleat.

My old cleats actually came off ok once I'd picked the dirt out of the hexagonal sockets and could get a key in there. And once off I could compare them with the new ones just like in that photo and lo-and-behold they were a millimetre or so shorter.

Unfortunately I picked an icy day to have brand new cleats with slightly tightened pedals and the combination contributed to my literal downfall as I skidded off the track into a hedge - I was fine and upright at this point - failed to unclip quickly enough and promptly fell over sideways.