Credits do not rollover; they expire 1 month after purchase, whether they were bought as part of your package or as additional credits.  

For example, if you buy a system with 5000 credits per month on January 1st, any unused credits will expire on February 1st and you will have a new balance of 5000.  If you buy additional credits at any time, they last for one month from their own unique purchase date. So, in the previous example, if you chose to buy an extra 500 credits on January 28th, they would remain in your balance until February 28th.