Freecash Coins do not expire. If you have earned Freecash Coins by completing offers, then per our current rules, you can let them sit on your account while you keep earning.
We may adjust the rules around Coins in the future, but if we do so, our users will be notified. You can find the current rules regarding Freecash Coins in our Terms of Service.
Rewards Expiration (§12): We may also adopt or modify our Rewards' expiration rules and policies for any past, present, or future Rewards from time to time, and if your Rewards expire pursuant to such then-current rules or policies, we may remove such Rewards from your Account, without any compensation or further obligation to you regarding the expired Rewards.
So, as long as your account is active, your coins are safe.
What if my account has been deemed inactive?
Although your Freecash Coins do not expire, your account itself may be closed if you have not been logged in for over a year. If your account is deemed inactive and closed, you will lose all Rewards.
Inactive Accounts (§11): Any Account that has not been logged into and Rewards either received or redeemed for one (1) year or more may be deemed inactive and the Account closed. We may also modify our inactive Account rules and policies in our Rewards Programs from time to time, and if your Account becomes inactive pursuant to such then-current rules or policies, we may close your Account accordingly, without any compensation or further obligation to you regarding the closed Account.
To avoid having your account deemed inactive, login to your account at least once a year.