Credits are valid for 1 month for Monthly Plans and 12 months for Annual Plans.
If you run out of credits before the end of the term (monthly/annually) you must upgrade your current plan to continue with the services, otherwise the service will be suspended until the term of the plan is renewed.

Unused credits can be carried over to different plans, either when upgrading or downgrading. The validity of the credits (1 or 12 months) depends on the current active plan. Ex. You made a plan purchase of $79/month (annual), so you have a total of $948 in credits valid for 12 months. Three (3) months later you have a balance of $500 but decide to upgrade to $499/month (monthly). The $500 credits inherited from the previous plan will now be governed by the terms of the active plan, which is 1 month.