Please how long does the transit visa last for

Hi,
I'm a national of Nigeria. I'm currently in Australia with a study visa. I will be travelling to China in November through Hong Kong. Also, I will be travelling to Nigeria in December with a lay over in Hong Kong. I know I would have to get a transit visa for the trip. Please how long does the transit visa last for, as I would want to use the visa for the both trips. I don't want to request for a transit visa twice.
Thank you,
Ure.