Yes, They must be used during the 30-days that your contract includes.
However, the hours which have not been spent along the month can be calculated and converted into money (with the equivalences formula), and the amount of money which was not invested, can be discounted for the next month suscription.
We suggest to start with a small (40 hours per month) package, so you can test and avoid have unused hours at the end of the month.