Generally for households a billing period will be 90 days or 3 months (Victorian residential gas customers receive their bills every 2 months). 

Due to various reasons, the period covered by your bill may have changed. This could be because you have just changed retailers, perhaps as a result of moving house.  Usually in this instance it is only the first bill that covers an extended period. If the billing period is extended, then your bill will be higher.  Conversely, if the billing period is shortened, then the bill amount will be lower.

The number of "bill days" is printed on your bill.