Your hair will grow naturally in that amount of time. You don't need to do anything in particular to help it grow.
How can I grow my hair out in less than 5 months?
Hm. Depends on how long you want it. There is a certain rate at which the hair grows and you simply cannot affect it. On the other hand, you can make sure your hair stays healthy and provide it with nutrients and minerals. Good shampoo and conditioners are the best way to do this. Diet is a tougher one, but a balanced diet affects every aspect of your body.
If you decide to go over 5 months, I'd suggest cutting the tips 2-3 times per year. This seems to speed things up a bit. :)