If you do not hold a stock for 12 continous months, then when...

If you do not hold a stock for 12 continous months, then when you sell, you have to declare all of the gain. This is simply added to your income at tax time and is taxed at your marginal rate.
If you hold for more than 12 months, you only have to declare half of the gain, and again it is added to your income and taxed at your marginal rate.
With any losses, you offest it against any capital gain (irrespective of how long it took you to lose that money.)
Losses can be accumulated and can be used years down the track if necessary once you make a net capital gain for the financial year.
Good luck, this market is going to be tough for a while... not a great time for newbies , take care.

cheers

Commy