By default, your Office 365 users will be required to change their password every 90 days. However, if you are an administrator for your Office 365 account and you are not using Active Directory synchronization, you can easily modify this expiration policy.
Once you log in to the Office 365 portal, click on the tile labeled admin, then click on Security & privacy under Settings on the left side. Click Edit next to Password policy, then enter the number of days you would like before your passwords expire. Note that this value must be between 14 and 730 days.
To set user passwords never to expire is possible as well, but requires the use of Windows PowerShell.