Your purchased credits don't expire.
Earned credits (from videos or offers) do expire. Make sure to not let your credits expire!
Your credits will expire if your account remains inactive for 180 days.
To be "active" you just need to earn credits (by completing offers) or to spend credits (by calling or sending SMS).
As long as you do any one of those things we will consider you are active and the 180-day counter will reset!