Update: This is now available via the Office 365 administration portal:

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If 90 days seems too short of a period before your Office 365 users’ passwords expire, you can change it via a powershell cmdlet.

Set-MsolPasswordPolicy -ValidityPeriod <days> -NotificationDays <days> -DomainName contoso.com

Validity Period: time before expiration

Notification Days: how many days prior to expiration that the user begins to receive notification pop-up

Make sure you use Powershell to connect to MS online service first: http://onlinehelp.microsoft.com/office365-enterprises/hh124998.aspx