Some varieties of radicchio naturally form into heads, for example Treveso. This is a non-forcing plant. In the summer the long 10-14" will be green then turn red as the weather cools. This variety does not require you to cut it back to get it to form the head.
Red Verona, a forcing variety needs a little assistance to form the classic round heads. This variety should not be planted too early in spring or it may bolt. In the fall, you need to cut all the leaves off above the crown. Then as the weather cools the plant will produce the small round heads.
Radicchio matures in approximately 3 months. It should be picked when it is mature but not old. The older it gets the tougher and more bitter the vegetable will be.
Grilled Radicchio Treviso With Scamorza Cheese
by Gourmet Magazine, September, 2000 (epicurious.com)