Maybe different brokers have different policies. I

use scottrade. So the best way is to call them or go to their office to see them. Nobody told me that I should keep my positions for one day and, if not, they will charge me of 10%.
Some limits are: If you have less than $25000, then you are only allowed to do 3 daytrade in every 5 business days. (Day-trade means to buy it and sell it on the same day). But if you have more than $25000, they will remove those limits for you, i.e, you are allowed to do as many as daytrades as you like.
Call them or see them and make the policy clear.