Did you get a notification saying your team is about to expire? Or maybe you're just curious to know if your team has an expiration date.
If your team name has a red expiration icon next to it, then it'll expire in fewer than 30 days. Go to the team name, and select More options 
> Renew now.
In other cases, either select the notification to renew your team, or go to your team settings. Go to the team name, and select More options > Manage team. Then select Settings > Team Expiration.
From there, you can select Renew now, or just check for the team's expiration date.