When Topping your plants?

Ok so i am on my first grow and i am just wondering if i top the tops what type of growth will i get from them. Say I top them at three feet tall (smaller area to grow in) how much taller do you think they will get. I have heard that when you top your plants it helps to keep them short but i was wondering how short it will keep them? Can anyone give me some more information on this topic.
Thanks for any help.

some people say they can grow as much as 3 times that. but id say top at 3 feet ull get like 5-6 maybe

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There is no set place or time as to when you top em, As long as you do not do it in flowering it will be cool , I usually just nip of the top nodd and the growth swells out giving multi colas , You can make any mistakes bro just nip of the topp..H4F