Here's a good post for topping/cropping info:

http://boards.cannabis.com/showthread.php?t=87399
Zoidberg Reviewed by Zoidberg on . Whens a good time to top a plant? My plants are on their 5th week of veging, i am going to have them in vegation for one more week . When should i top the plants? They are 13-14 inches tall right now. PS is that an okay size for right now? And can any body tell me the avarge yeild for a 2' 6'' plant? Rating: 5