Set Booking Window for all Room Mailboxes with powershell.
Connect to Exchange Online with powershell
$session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri "https://ps.outlook.com/powershell/" -Credential $cred -Authentication Basic -AllowRedirection
Import-PSSession $session
get-mailbox|where {$_.recipientTypeDetails -eq "roomMailbox"} | Set-CalendarProcessing -BookingWindowInDays 365
To check the windows:
get-mailbox|where {$_.recipientTypeDetails -eq "roomMailbox"} | Get-CalendarProcessing | FL Identity,BookingWindowInDays