I spot feed mine about 2 times a week without really keeping track and they are growing and doing well. But I don't worry about it if I can't spot feed them for a week or two. One time a few months ago I was busy with life and hadn't fed them for a couple weeks and they started to recede, but then I fed them again and they perked up.
Also I had a long spine urchin who decided to munch away about 1/3 of one of the heads and I worried it was a goner because it wouldn't fully extend after that, but after a few weeks of making a point of more frequent spot feeding of it, it was fine again, just a bit flatter on one side.